Ultra-high strain responses in lead-free (Bi0.5Na0.5)TiO3-BaTiO3-NaNbO3 ferroelectric thin films

Lead-free (Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoelectric materials, have a great potential for high-precision actuators’ applications. In this work, the high-quality (0.94-x%)(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-x%NaNbO₃ (x = 2–10, BNT-6BT-xNN) thin films have been successfully deposited on Pt/TiO₂/SiO₂/Si substrates by sol-gel method. An ultra-high poling strain S_pol value of 1.7% with a unipolar strain S_uni value of 1.47% was reported in the BNT-6BT-6NN thin films. The coexistence of the ferroelectric phase and relaxor state was observed in the compositions of x = 2–8. Furthermore, the BNT-6BT-6NN thin films show more active domain switching compared to other compositions. It is demonstrated that the optimized strain responses in the BNT-6BT-6NN are attributed to a synergistic reaction of active domain switching and reversible electric-field-induced phase transition between the ferroelectric phase and relaxor state. Our systematic study demonstrates that the BNT-6BT-xNN thin films with an improved strain response are promising candidates for the applications of miniaturized actuators.     

Large strain and low hysteresis in (0.64-x) Bi0.5Na0.5TiO3- 0.36Sr0.7Bi0.2TiO3- x(K0.5La0.5)(Ti0.9Zr0.1)O3 piezoceramics

In this work, (0.64-x)Bi_0.5Na_0.5TiO₃- 0.36Sr_0.7Bi_0.2TiO₃- x(K_0.5La_0.5)(Ti_0.9Zr_0.1)O₃ lead-free piezoceramics were designed and fabricated by a conventional solid-phase sintering process. It is found that large strains (0.33 %), low hysteresis coefficients (32 %), and large dynamic d₃₃* (367 p.m./V) were obtained at x = 0.01. The large strain originates from the reversible transition of the relaxor to the long-range ferroelectric order in the electric field. When the ferroelectric and relaxor phases coexist in a proper ratio, they can provide a favorable condition for the easier movement of the domains and improve the strain properties. In addition, after 10⁵ cycles, the bipolar strain loop of x = 0.01 content changed slightly, demonstrating excellent fatigue resistance. This work provides a new way to design piezoelectric ceramics with large strain and low hysteresis.     

Lead-free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage

A series of (1-x)(0.65BaTiO₃-0.35Bi_0.5Na_0.5TiO₃)-xNa_0.73Bi_0.09NbO₃ ((1-x)BBNT-xNBN) (x = 0–0.14) ceramics were designed and fabricated using the conventional solid-state sintering method. The microstructure, dielectric property, relaxor behavior and energy storage property were systematically investigated. X-ray diffraction results reveal a pure perovskite structure and dielectric measurements exhibit a relaxor behavior for the (1-x)BBNT-xNBN ceramics. The slim polarization electric field (P-E) loops were observed in the samples with x ≥ 0.02 and the addition of Na_0.73Bi_0.09NbO₃ (NBN) could decrease the remnant polarization (P_r) of the (1-x)BBNT-xNBN ceramics obviously. The sample with x = 0.08 exhibits the highest energy storage density of 1.70 J/cm³ and the energy storage efficiency of 82% at 172 kV/cm owing to its submicron grain size and high relative density. These results show that the (1-x)BBNT-xNBN ceramics may be promising lead-free materials for high energy storage density capacitors.     

Enhanced energy storage in temperature stable Bi0.5Na0.5TiO3-modified BaTiO3-Bi(Zn2/3Nb1/3)O3 ceramics

Recently, BaTiO₃-BiMeO₃ ceramics have garnered focused research attention due to their outstanding performance, such as thermal stability, energy efficiency and rapid charge-discharge behavior, however, a lower recoverable energy storage density (W_rec) caused by a relatively low P_max (<30 μC/cm²) mainly hinders practical applications. Herein, the energy density and thermal stability are improved by adding a tertiary component, i.e., Bi_0.5Na_0.5TiO₃, into BaTiO₃-BiMeO₃, resulting in xBi_0.5Na_0.5TiO₃-modified 0.88BaTiO₃-0.12Bi(Zn_2/3Nb_1/3)O₃ ceramics, with x = 0, 0.1, 0.2, 0.3 and 0.4, with superior dielectric properties and eco-friendly impact. Incorporating Bi_0.5Na_0.5TiO₃ with a high saturation polarization and Curie temperature not only significantly enhances P_max of BaTiO₃-Bi(Zn_2/3Nb_1/3)O₃ but also improves Curie temperature of (1-x)[0.88BaTiO₃-0.12Bi(Zn_2/3Nb_1/3)O₃]-xBi_0.5Na_0.5TiO₃ system. Combined with complementary advantages, modified ceramics render a superior energy storage performance (ESP) with a high W_rec of 3.82 J/cm³, efficiency η of 94.4% and prominent temperature tolerance of 25–200 °C at x = 0.3. Moreover, this ceramic exhibit excellent pulse performance, realizing discharge energy storage density W_dis of 2.31 J/cm³ and t_0.9 of 244 ns. Overall, the proposed strategy effectively improved comprehensive properties of BaTiO₃-based ceramics, showing promise in next-generation pulse applications.     

Tailoring Strain Properties of (0.94−x)Bi1/2Na1/2TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 Ferroelectric/Relaxor Composites

A remarkable progress in the quest of lead‐free piezoceramics for actuator applications has been made with the development of incipient piezoceramics featured by giant strains. A drawback, however, is the high electric field required to generate this giant strain. A powerful approach to overcoming this drawback lies in relaxor/ferroelectric (FE) composites comprised such giant strain materials (matrix) and a FE or nonergodic relaxor (seed). In this study, we investigate the effect of K_0.5Na_0.5NbO₃ content in the matrix and the volume ratio of seed to matrix using composites of 0.93Bi_1/2Na_1/2TiO₃–0.07BaTiO₃ as a seed and (0.94 ? x)Bi_1/2Na_1/2TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃ as a matrix. The strain of all matrices, independent of their K_0.5Na_0.5NbO₃ content, was found to be enhanced by adding a certain amount of seed. An optimum strain is achieved for the composite comprised of a matrix with x = 0.02 K_0.5Na_0.5NbO₃ and 10% seed. By means of a differential analysis on the temperature‐dependent dielectric permittivity, it was shown that the seed phase is still present in the composites despite the naturally expected diffusion process during sintering.     

Electrical properties and temperature stability of SrTiO3-modified (Bi1/2Na1/2)TiO3-BaTiO3-(K1/2Na1/2)NbO3 piezoceramics

SrTiO₃-modified lead-free piezoelectric ceramics, (0.93-x)Bi_0.5Na_0.5TiO₃-xSrTiO₃-0.06BaTiO₃-0.01 K_0.5Na_0.5NbO₃ [(BNT-xST)-BT-KNN, x = 0-0.06], were prepared using a conventional solid-state reaction method. The XRD structure analysis and electric properties characteristics revealed the ST-induced phase transformation from the ferroelectric phase to the relaxor phase and their coexistence state. Benefiting from the ST-destructed ferroelectric long-range orders, the high normalized strain value of 600 pm/V was obtained in the (BNT-0.02ST)-BT-KNN ceramic at 5 kV/mm. The ST-generated relaxor phase was found to have a constructive effect on improving the temperature stability and restraining the hysteresis of the electric-field-induced strain. The normalized strain of (BNT-0.06ST)-BT-KNN ceramics could be kept at a high value ~337 pm/V at elevated temperature up to 120°C.     

Electrostatic energy storage performances of La(Ni2/3Ta1/3)O3-modified Na0.5Bi0.5TiO3 lead-free ceramics

Ceramic capacitors with high electrostatic energy storage performances have captured much research interest in latest years. Sodium bismuth titanate (Na_0.5Bi_0.5TiO₃)-based ferroelectric ceramics show great potential due to their environment-friendly composition, high polarization, and excellent relaxor properties. However, the nonergodic relaxor state of Na_0.5Bi_0.5TiO₃-based ceramics hampers the decrement of remanent polarization, leading to poor energy storage performance. Herein, the (1 − x)Na_0.5Bi_0.5TiO₃–xLa(Ni_2/3Ta_1/3)O₃ ceramics were designed to generate the transformation between nonergodic and ergodic relaxor state. As a result, the ceramics exhibit improved dielectric relaxation, slim polarization–electric field loops, and flattened current–electric field curves due to highly dynamic polar nanoregions. Particularly, the 0.85Na_0.5Bi_0.5TiO₃–0.15La(Ni_2/3Ta_1/3)O₃ ceramics show large breakdown electric field E_b (345 kV/cm), high recoverable energy density W_rec (3.6 J/cm³), and efficiency η (80.6%), revealing potential applications in electrostatic energy storage.     

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.     

Na0.5Bi0.5TiO3-based ferroelectric relaxor with high thermal stability and anti-fatigue for charge-discharge properties

Apart from discharge energy density (W_r) and discharging time (t_0.9), thermal stability and anti-fatigue for charge-discharge performance are also the important performance indexes for dielectric pulsed capacitor. Na_0.5Bi_0.5TiO₃ based ceramics are usually accompanied by huge electric field-induced strain when appling electric field, resulting in the fatigue phenomenon and thermal accumulation effect in the cycling process. In this work, Na_0.5Bi_0.5TiO₃-xNaNbO₃ (NBT-xNN) ferroelectric relaxor ceramic has been prepared by the solid state reaction process. The effect of NaNbO₃ content on microstructures, impedance spectroscopy, electric-field-induced strain and charge-discharge performance of NBT-xNN ceramics have been investigated systematically. Results indicate the proper percent of NaNbO₃ could favor the formation of polar nanoregions (PNRs), which leads to the diffusion of phase transition and the diminution of electromechanical strain. Therefore, the high thermal stability and anti-fatigue for charge-discharge property has been achieved in NBT-xNN ceramics. An enhanced discharging energy density of 2.44 J cm^?3 along with discharge time of 0.31 μs could be obtained in the NBT-xNN with x = 0.3, and a very stable discharge energy density of 2.06 J cm^?3 concomitantly with discharge time less than 0.37 μs could be gained in a wide temperature range of 20–150 °C with a fluctuation of ±4% after 10⁴ charge/discharge cycles. This work would contribute to the development of charge-discharge system, especially dielectric capacitor, for green pulsed power devices.     

Electro-mechano-optical properties of the Er3+ modified Bi0.5Na0.4K0.1TiO3 versatile ceramics

A series of Bi_0.5-xEr_xNa_0.4K_0.1TiO₃ (BNKT-xEr) ceramics were designed and fabricated, the dopant effects on dielectric, piezoelectric and photoluminescence properties were studied. The results show that the piezoelectric property of BNKT can be enhanced by a trace amount of Er dopant, which is also reflected in the large linear electrostrain (S_uni = 0.29%, under 55 kV/cm) achieved in BNKT-0.0025 Er. On the other hand, higher Er content can produce excellent dielectric temperature stability, with △?/?_{150 °C} < ±15% over temperature range of 90～510 °C. Meanwhile, all BNKT-xEr ceramics exhibit good photoluminescence properties, which may open new applications of these multifunctional ceramics.     

Nb-doped BaTiO3-(Bi0.5Na0.5)TiO3 ceramics with core-shell structure for high-temperature dielectric applications

Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ temperature-insensitive ceramics with novel core-shell structure were sintered at low temperature by the conventional solid-state reaction method. The beneficial role of Nb in facilitating the formation of core-shell structure because of chemical inhomogeneity is verified, which is responsible for the weak temperature dependence of dielectric properties. Temperature dependence of permittivity measured at different frequencies shows high frequency dispersion at low temperature, while without relaxor characteristic at high temperature. The Vogel–Fulcher model was adopted to study the relaxor behaviour of Nb-doped 0.90BaTiO₃-0.10(Bi_0.5Na_0.5)TiO₃ ceramics at low temperature. The samples with an addition of 1.5?mol% Nb₂O₅ provide a temperature coefficient of capacitance meeting the requirements of the X9R characteristic, and result exhibits an optimum dielectric behaviour of ε_r ～1900, tanδ ～1.8% at room temperature, making the material a promising candidate for high temperature applications.     

Two-step sintering of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 lead-free piezoelectric material

Two-step sintering (TSS) as an efficient sintering method for obtaining dense microstructure while preventing excess grain growth was used for sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ composition which is located near the morphotropic phase boundary of this binary system. In order to compare the obtained microstructure and piezoelectric properties, conventional single step sintering (SSS) was also examined. Microstructure evolution during sintering at different temperatures was investigated to find the optimum sintering temperature. Ferroelectric hysteresis loop as well as unipolar strain behavior of optimally sintered ceramics was studied. According to density measurement and microstructure studies of the prepared ceramics, TSS resulted in finer and more dense and uniform microstructure compared to SSS method. As a result remnant polarization of TSSed ceramic was increased by 35% and its coercive field was decreased by 16%. The inverse piezoelectric coefficient of the SSSed and TSSed was obtained 220 and 300 p.m./V, respectively. These values are high enough for practical applications such as actuators. The obtained results clearly showed that TSS is capable of sintering 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ at temperatures lower than which is required for SSS method. Therefore the composition stoichiometry is maintained after sintering and denser microstructure without abnormal grain growth is obtained which is responsible for improved electrical properties of the piezoceramics.     

High energy storage properties of lead-free Mn-doped (1-x)AgNbO3-xBi0.5Na0.5TiO3 antiferroelectric ceramics

In this work, 0.2 wt.% Mn-doped (1-x)AgNbO₃-xBi_0.5Na_0.5TiO₃ (x = 0.00–0.04) ceramics were synthesized via solid state reaction method in flowing oxygen. The evolution of microstructure, phase transition and energy storage properties were investigated to evaluate the potential as high energy storage capacitors. Relaxor ferroelectric Bi_0.5Na_0.5TiO₃ was introduced to stabilize the antiferroelectric state through modulating the M₁-M₂ phase transition. Enhanced energy storage performance was achieved for the 3 mol% Bi_0.5Na_0.5TiO₃ doped AgNbO₃ ceramic with high recoverable energy density of 3.4 J/cm³ and energy efficiency of 62% under an applied field of 220 kV/cm. The improved energy storage performance can be attributed to the stabilized antiferroelectricity and decreased electrical hysteresis ΔE. In addition, the ceramics also displayed excellent thermal stability with low energy density variation (<6%) over a wide temperature range of 20−80 °C. These results indicate that Mn-doped (1-x)AgNbO₃-xBi_0.5Na_0.5TiO₃ ceramics are highly efficient lead-free antiferroelectric materials for potential application in high energy storage capacitors.     

Giant electro-strain and enhanced energy storage performance of (Y0.5Ta0.5)4+ co-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 lead-free ceramics

0.94(Bi_0.5Na_0.5)(Y_0.5Ta_0.5)_xTi_1-xO₃-0.06BaTiO₃ lead-free piezoelectric ceramics were prepared by a conventional solid-state reaction method to study their excellent electro-strain properties and energy storage characteristics systematically. All ceramics exhibited a dense surface morphology. The introduction of (Y_0.5Ta_0.5)⁴⁺ complex ions destroyed the long-range ferroelectric order, which reduced the T_F-R to the operating temperature, resulting in an easier field-induced transition between relaxor and ferroelectric phase. Therefore, for x = 0.01 component attained unipolar strain of 0.37% under 52 kV/cm, and the corresponding normalized strain d₃₃* was 708 pm/V. Besides, the destruction of the ferroelectric phase also induced a pinched hysteresis loop and the maximum storage density of 1.215 J/cm³ with the efficiency of 68.7% obtained at 98 kV/cm for BNYT30 ceramics. These all demonstrated that the doping of complex ions (Y_0.5Ta_0.5)⁴⁺ made the BNT-BT an outstanding candidate for actuators and energy storage devices.     

Enhanced optical transmittance and energy-storage performance in NaNbO3-modified Bi0.5Na0.5TiO3 ceramics

Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb-free energy-storage ceramics were rare reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xNaNbO₃ (BNT–xNN) by conventional solid-state reaction method. We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺ to Ti³⁺. As a result, a high recoverable energy-storage density of 5.14 J/cm³ and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN-doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy-storage dielectric ceramics.     

Quenching-circumvented ergodicity in relaxor Na1/2Bi1/2TiO3-BaTiO3-K0.5Na0.5NbO3

Quenching alkaline bismuth titanates from sintering temperatures results in increased lattice distortion and consequently higher depolarization temperature. This work investigates the influence of quenching on the ergodicity of relaxor Na_1/2Bi_1/2TiO₃-BaTiO₃-K_0.5Na_0.5NbO₃. A distinct departure from ergodicity is evidenced from the increase in remanent polarization and the absence of frequency dispersion in the permittivity response of poled samples. Further, the samples exhibit enhanced negative strain upon application of electric field, indicating proclivity towards correlated polar nanoregions, corroborated by the enhanced tetragonal distortion. As a result, ergodic relaxor Na_1/2Bi_1/2TiO₃-6BaTiO₃-3K_0.5Na_0.5NbO₃ exhibits a depolarization temperature of 85°C with a 60% increase in remanent polarization and approximately a threefold increase in remanent strain upon quenching. Quenching-induced changes in the local environment of Na⁺ and Bi³⁺ cations hinder the development of ergodicity promoted by the A-site disorder. These results provide new insight into tailoring ergodicity of relaxor ferroelectrics.     

Dielectric and ferroelectric properties of (1 − x)BaTiO3–xBi0.5Na0.5TiO3 ceramics

Lead-free (1 − x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (x = 0.01, 0.02, 0.05, 0.1, 0.2, 0.3) ferroelectric ceramics were fabricated by the conventional ceramic technique. Sintering was made at 1200 °C for 2–4 h in air atmosphere. The crystal structure was investigated by X-ray diffraction. The dielectric and ferroelectric properties were also studied. Room temperature permittivity was found to decrease as Bi_0.5Na_0.5TiO₃ (BNT) content increases. Only the sample with 0.3 mol BNT was found to have relaxor behaviour. The T_c shifted slightly only for BNT addition lower than 0.1 mol. The highest T_c (about 150 °C) was obtained for 0.2 mol BNT addition. The remanent polarization, P_r, decreases whereas the coercive field, E_c, increases monotonously as the BNT content increases.     

Synthesis and characterizations of BNT–BT and BNT–BT–KNN ceramics for actuator and energy storage applications

Dielectric and ferroelectric properties of 0.93Bi_0.5Na_0.5TiO₃–0.07BaTiO₃ (BNT–BT) and 0.93Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–0.01K_0.5Na_0.5NbO₃ (BNT–BT–KNN) ceramics were studied in detail. An XRD analysis confirmed the single perovskite phase formation in both the samples. Room temperature (RT) dielectric constant (ε_r) ~1020 and 1370, respectively at 1 kHz frequency were obtained in the BNT–BT and BNT–BT–KNN ceramics. Temperature dependent dielectric and the polarization vs. electric field (P–E) studies confirmed the coexistence of ferroelectric (FE) and anti-ferroelectric (AFE) phases in the BNT–BT and BNT–BT–KNN ceramics. Substitution of KNN into the BNT–BT system decreased the remnant polarization, coercive field and the maximum strain percentage. The energy storage density values ~0.485 J/cm³ and 0.598 J/cm³ were obtained in the BNT–BT and BNT–BT–KNN ceramics, respectively. High induced strain% in the BNT–BT ceramics and the high energy storage density in the BNT–BT–KNN ceramics suggested about the usefulness of these systems for the actuator and the energy storage applications, respectively.     

An alternative way to design excellent energy-storage properties in Na0.5Bi0.5TiO3-based lead-free system by constructing relaxor dielectric composites

With growing concerns over environmental protection, lead-free dielectric ceramic capacitors are attracting much attention. In this work, a series of novel (1-x) Na_0.5Bi_0.5TiO₃-x Ba₅LaTi₃Ta₇O₃₀ ((1-x)NBT-xBLTT) dielectric composite ceramics were fabricated by a traditional solid‐state method. All the samples possess a compact microstructure with refined grain morphology with increasing BLTT content, and tend to exhibit a diphase dielectric composite as x reaches up to 0.05. Furthermore, the addition of BLTT enhances the dielectric relaxor behavior of NBT-based ceramics, such that the x = 0.15 composite ceramic exhibits a typical feature of relaxor ferroelectrics. As a result, a high recoverable energy-storage density of W_rec~3.67 J/cm³, an ultrahigh energy-storage efficiency of η~97.3%, and a high power density of P_D~333 MW/cm³ can be simultaneously obtained in the x = 0.15 relaxor composite ceramic. This study provides an alternative way to design excellent energy-storage performances in NBT-based compositions through constructing dielectric relaxor composites via introducing non-polar tungsten bronze oxides.     

Dielectric Relaxor Evolution and Frequency‐Insensitive Giant Strains in (Bi0.5Na0.5)TiO3‐Modified Bi(Mg0.5Ti0.5)O3–PbTiO3 Ferroelectric Ceramics

The 0.45Bi(Mg_0.5Ti_0.5)O₃–(0.55 ? x)PbTiO₃–x(Bi_0.5Na_0.5)TiO₃ (BMT–PT–xBNT) ternary solid solution ceramics were prepared via a conventional solid‐state reaction method; the evolution of dielectric relaxor behavior and the electrostrain features were investigated. The XRD and dielectric measurements showed that all studied compositions own a single pseudocubic perovskite structure and undergo a diffuse‐to‐relaxor phase transition owing to the evolution of the domain from a frozen state to a dynamic state. The formation of the above dielectric relaxor behavior was further confirmed by a couple of measurements such as polarization loops, polarization current density curves, as well as bipolar strain loops. A large strain value of ~0.41% at a driving field of 7 kV/mm (normalized strain d₃₃* of ~590 pm/V) was obtained at room temperature for the composition with x = 0.32, which is located near the boundary between ergodic and nonergodic relaxor. Moreover, this electric field‐induced large strain was found to own a frequency‐insensitive characteristic.