Effect of Texture on Temperature‐Dependent Properties of K0.5Na0.5NbO3 Modified Bi1/2Na1/2TiO3–xBaTiO3

Textured (1?x?y)Bi_1/2Na_1/2TiO₃–xBaTiO₃–yK_0.5Na_0.5NbO₃ (BNT–100xBT–100yKNN) ceramics with a {001} pseudocubic (pc) orientation were fabricated by templated grain growth using Bi_1/2Na_1/2TiO₃ templates. Temperature‐dependent electromechanical results demonstrate that the strain response of templated BNT–xBT–yKNN ceramics is stable from room temperature (RT) to 125°C. The temperature‐dependent strain and polarization response are compared to randomly oriented ceramics, for BNT–100xBT–2KNN (0.05 ≤ x ≤ 0.07). Textured BNT–7BT–2KNN reached a maximum 0.47% strain response at 5 kV/mm, an almost 50% increase compared to randomly oriented BNT–7BT–2KNN. Over the temperature range RT–125°C, the strain response of templated BNT–6BT–2KNN degraded from 0.38% to 0.22% (?42.1%) compared to 0.37% to 0.18% (?51.4%) for randomly oriented ceramics. The temperature‐dependent strain response suggests that templated BNT–100xBT–100yKNN ceramics are well suited for elevated temperature applications.     

Large electric-field-induced strain in B-site complex-ion (Fe0.5Nb0.5)4+-doped Bi1/2 (Na0.82K0.12)1/2TiO3 lead-free piezoceramics

In order to obtain a new system of (Bi_1/2Na_1/2)TiO₃ (BNT) based lead-free incipient piezoceramics with large strain for practical applications of actuators, we investigated the effect of B-site complex-ion (Fe_0.5Nb_0.5)⁴⁺ (FN)-doped Bi_1/2 (Na_0.82K_0.12)_1/2TiO₃ ceramics on the phase structure, dielectric, ferroelectric, piezoelectric and electric-field-induced strain properties. All samples exhibited single perovskite phase with pseudocubic symmetry. The room temperature electric-field-induced polarization (P-E) and strain (S-E) hysteresis loops indirectly illustrated ferroelectric-to-relaxor (FE-RE) phase transition. The increasing content of FN doping decreased the FE-RE phase transition temperature, T_F-R to below room temperature and induced the reversible FE-RE phase transition, giving rise to a large strain of 0.462% with a normalized strain, d^*₃₃ of 660 pm/V at a critical composition of x = 5. A fluctuation of the dielectric curve for BNKT-5 mol% FN ceramics in the spectra around 80 °C before and after polarization suggested that the large strain response can be induced via delicate mixing of the FE and RE phase.     

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.     

Contrasting phenomena of quenching-induced piezoelectric performance in (0.4Na1/2Bi1/2TiO3-0.6BiFeO3)-xBaTiO3 ferroelectrics and relaxors

Na_1/2Bi_1/2TiO₃ (NBT)-based materials are promising lead-free alternatives due to their large electrostrain and stable mechanical quality factor. Nonetheless, the relatively low depolarization temperature (T_d) impairs its practical application. Recently, quenching from sintering temperature was adopted to increase T_d of NBT-based ceramics. However, the origin of the quenching-induced increase in T_d is still debated. In this study, quenching effects in (1-x)(0.4Na_1/2Bi_1/2TiO₃-0.6BiFeO₃)-xBaTiO₃ ceramics are investigated. With increasing BaTiO₃ content, this system transforms from ferroelectric to relaxor state at room temperature, with a criticality at x = 0.07, which exhibits R3c and P4bm coexisted phases. Ferroelectric and relaxor compositions exhibit different responses upon quenching. Upon quenching the ferroelectrics, T_d increases from 420 to 580 °C for x = 0.04, but d₃₃ is majorly unaltered. However, upon quenching the relaxors, T_d increases marginally, while d₃₃ increases from 62 to 97 pC/N. The correlation between the structural evolution and electrical responses upon quenching ferroelectric and relaxor compositions is explored.     

Advances in mitigating the Td-d33 trade-off via compositionally graded diffusion in BNT-based piezoceramic

A notoriously unresolved conflict that the substantial increment of depolarization temperature T_d was generally accompanied by the sharply deterioration of piezoelectric constant d₃₃ in Bi_0.5Na_0.5TiO₃-based (BNT-based) piezoceramics severely limit their practical applications. Herein, a new strategy by Ba²⁺ gradient diffusion layer in 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (BNT-6BT) ceramic via interface-diffused sintering to suppress the ferroelectric-relaxor phase transition is presented to manipulate the incompatibility of T_d and d₃₃. By this strategy, the compositionally graded diffusion in the BNT-6BT ceramic can offer both the properties simultaneously with the T_d deferring from 109 °C to 170 °C and maintaining the initial d₃₃ (145pC/N). Furthermore, by integrating the interface-diffused sintering and quenching process, the T_F-R is significantly deferring from 115 °C to 214 °C while the d₃₃ * is retaining the initial value (d₃₃ *=S_max/E_max=258 pm/V). Diffusion phase transition caused by Ba²⁺ concentration gradient promotes the greatly increased tetragonal phase ratio and the development of built-in field thereby leading to well-balanced performance between T_d and d₃₃ in BNT-6BT ceramic. Our work opens a new avenue for developing the high-performance lead-free BNT-based piezoceramics.     

Enhanced electrical properties of Cr2O3 addition NBT-based high-temperature piezoelectric ceramics

Owing to industrial and technological developments, there has been an increasing demand for piezoelectric ceramics that can function at temperatures of 500°C or higher. Na_0.5Bi_4.5Ti₄O₁₅ (NBT) with its high Curie temperature (T_C) of 650°C is a typical bismuth layer–structured ferroelectric. However, its relatively low piezoelectric coefficient (d₃₃ ∼ 16 pC/N) hinders its potential application at high temperatures. In this study, compositions of Ca_0.05(Na_0.5Bi_0.5)_0.95Bi₄Ti₄O₁₅ with different additions of Cr₂O₃ (CNBT–Cr100x) were designed based on previous studies on Ca²⁺-doped NBT piezoceramics, and the effects of the addition on the structural and electrical properties were investigated. The d₃₃ value of CNBT–Cr20 was as high as 29 pC/N, almost twice higher than that of pure NBT ceramics. This increase was investigated in depth using X-ray diffraction refinement and piezoelectric force microscopy in terms of intrinsic and extrinsic contributions. The P_s values of CNBT and CNBT–Cr20 were almost equal. The density of the domain walls of CNBT–Cr20 was significantly higher than that of CNBT, indicating that the increase of d₃₃ of CNBT–Cr20 is mainly due to the increase in the extrinsic contribution. The CNBT–Cr20 ceramic exhibited excellent properties with a high T_C of 655°C, a high d₃₃ of 29 pC/N, and a resistivity high than 10⁶ Ω cm at 500°C, demonstrating its potential for applications at high temperatures such as 500°C.     

Effect of quenching treatment on photochromic properties of Mg2+ doped Na1/2Bi1/2TiO3 ceramics

This study investigates the photochromic (PC) properties of Na_1/2Bi_1/2TiO₃ (NBT)-based ceramics. Through doping Mg²⁺ (x = 0.01, 0.02, 0.03, and 0.04) and quenching treatment, a large number of oxygen vacancies are introduced into the ceramic matrix, thereby improving its PC efficiency. Specifically, an appropriate amount of Mg²⁺ doping could significantly improve the PC contrast (ΔR) of NBT-based ceramics. When x = 0.03, ΔR reaches 26.9% at 500 nm and then further increases to 40.4% after quenching. Furthermore, the effects of grain size, the second phase, and the band gap on the photochromism behavior are also discussed. These factors offer a new way to improve PC performance.     

Large strain with low hysteresis in Bi4Ti3O12 modified Bi1/2(Na0.82K0.18)1/2TiO3 lead-free piezoceramics

In order to obtain BNT-based ceramic system with excellent electric-field-induced strain performance for actuator applications, a novel solid solution (100-x)Bi_1/2(Na_0.82K_0.12)_1/2TiO₃-xBi₄Ti₃O₁₂ ((100-x)BNKT-xBiT, x?=?0–12?wt%) was designed and fabricated by solid state synthesis. The microstructure and related electrical properties of this material were systematically investigated. It was found that BiT is dissolved into the lattice structure of the BNKT, leading to a greater increase in the volatilization of Na and K, thus produce more A-site vacancies compared with the undoped BNKT. The 9?wt%BiT doped sample not only has sufficient A-site vacancies to destroy the long-range ferroelectric order of the base composition but also favors the presence of extremely stable relaxor phase at room temperature. Further, the ferroelectric-to-relaxor phase transition temperature T_F-R can be effectively tuned to about 0?°C, giving rise to a large signal piezoelectric coefficient d^*₃₃ of 485?pm/V with a small hysteresis η of 23%.     

Crack‐tip toughness of lead‐free (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 piezoceramics

Crack opening displacements were evaluated on semi‐elliptical indentation cracks in lead‐free (1?x)(Na_1/2Bi_1/2)TiO₃–xBaTiO₃ piezoceramics and a commercially available PZT ceramic. The observed crack‐tip toughness of NBT‐xBT was found to be substantially higher than for PZT. Two evaluations for the crack opening displacement were demonstrated and contrasted: A more elaborate three‐term‐approximation and a pragmatic utilization of the Irwin parabola.     

In situ hot-stage TEM of the phase and domain evolution in quenched Na1/2Bi1/2TiO3–BaTiO3

Quenching relaxor ferroelectric 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ (NBT-6BT) enhances the depolarization temperature (T_d), linked to the stabilization of ferroelectric order. The thermal evolution of the domain structure and phase assemblage provides insights about the onset of ferroelectric order in quenched materials. Unpoled furnace cooled and quenched NBT-6BT ceramics were studied using in situ temperature-dependent transmission electron microscopy. The rhombohedral to tetragonal structural transition in furnace cooled and quenched samples occurs in a comparable temperature range of 120°C–220°C. While the tetragonal phase is characterized by polar nanoregions (PNRs) and no domain contrast in the furnace cooled state, the quenched composition exhibits an increased fraction of lamellar domains, which are partially stable up to 300°C, thus benefiting the delayed depolarization. This is further corroborated by the dielectric data indicating earlier freezing of PNR dynamics in the quenched state. The reversibility of the phase transition is demonstrated by successive cooling, where quenched NBT-6BT features an increased grainy PNR contrast after the experiment, followed by a kinetically delayed coalescence of PNRs back into lamellar domains. This demonstrates that the stabilized ferroelectric state upon quenching is associated with the conversion of polar units on the nanometer scale into long-range domain structures.     

Large electrocaloric effect with ultrawide temperature span in Na1/2Bi1/2TiO3-based lead-free ceramics

Innovative cooling technologies are recognized by many industries as a crucial part of their system design. A large electrocaloric effect (ECE) and extended working temperature are the key issues hindering the realization of electrocaloric refrigeration technology. In this work, large ECE (ΔT = 0.8–0.9°C @ 4 kV/mm) with an ultrawide temperature span from 30 to 120°C is noted for lead-free (Na_1/2Bi_1/2)_0.80Sr_0.20(Zn_1/3Nb_2/3)_xTi_1-xO₃ ceramics. The excellent ECE performance can be ascribed to the evolution of polar nanoregions. Our work suggests that this material is promising for applications in solid-state refrigeration systems with a broad range of operating temperatures.     

Macroscopic and Nanoscopic Polarization Relaxation Kinetics in Lead‐Free Relaxors Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3–BiZn1/2Ti1/2O3

The stability of the field‐induced ferroelectric (FE) state was studied in relaxor lead‐free ceramics (1 ? y)[0.81Bi_1/2Na_1/2TiO₃–0.19Bi_1/2K_1/2TiO₃]–yBiZn_1/2Ti_1/2O₃ both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn_1/2Ti_1/2)O₃, which are in the non‐ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two‐step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn_1/2Ti_1/2)O₃ content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature.     

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.     

Highly enhanced thermal stability in quenched Na0.5Bi0.5TiO3-based lead-free piezoceramics

Unquenched and quenched ceramics of 0.85Na_0.5Bi_0.5TiO₃-0.11K_0.5Bi_0.5TiO₃-0.04BaTiO₃ have been prepared, and their crystal structure, temperature-dependent ferro-/piezoelectric properties and domain structure have been comparatively investigated. It is shown that quenching process can significantly improve the ferroelectric-relaxor transition temperature (T_F-R), which is 130 °C for unquenched ceramics and 198 °C for quenched one. As the result, the thermal stability of ferro-/piezoelectric properties is highly enhanced. These observations are mainly attributed to the quenching induced stable rhombohedral ferroelectric phase and the defect altered domain evolution. This work may deepen the understanding of the effect of quenching on crystal structure, domain structure and their contributions to thermal stability of NBT-based ceramics.     

Modulation of electrostriction and strain response in bismuth sodium titanate‐based ceramics

The electrostriction and strain response of lead‐free Bi_0.5Na_0.5TiO₃–BaTiO₃ piezoceramics with La and Nb [(Bi_0.47Na_0.47Ba_0.06)_1?xLa_xTi_1?yNb_yO₃] were modified by optimizing the depolarization temperature. The influences of La and Nb on their phase structure and electrical properties were systematically investigated. All the ceramics exhibited a pseudocubic phase, which is independent of the addition of La and Nb. The strain values increased gradually with the addition of La, and a high strain of ~0.5% (80 kV/cm) was attained without any remnant strain when the compositions had a value of x = 0.015. Instead, the piezoelectric constant d₃₃ dropped down ~20 pC/N due to the shift of the T_d (or T_f?r) to room temperature. Interestingly, by establishing the relationship between T_d and strain values, it should be feasible to optimize the strain property of Bi_0.5Na_0.5TiO₃ (BNT)‐based ceramics by regulating T_d to ambient temperature. In addition, a very high electrostriction coefficient Q₃₃ of ~0.0758 m⁴ C^?2 can be found under high temperatures of 125 and 150°C. We believe that the strain and electrostriction behavior of BNT‐based ceramics can be well modified by the modulation of depolarization temperature.     

Large electric field-induced strain in ternary Bi0.5Na0.5TiO3-BaTiO3-Sr2MnSbO6 lead-free ceramics

The large electric-field-induced strain of Bi_0.5Na_0.5TiO₃-BaTiO₃ based ceramic make it a potential replacement for lead-based ferroelectrics in actuator applications. Herein, a ternary system (1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065TiO₃-xSr₂MnSbO₆ (BNBT6.5-xSMS) ceramic was fabricated using conventional solid-state reaction. It was found that the ferroelectric to relaxor phase transition temperature T_F-R gradually shifted to lower temperature by increasing SMS contents. The ferroelectricity and piezoelectricity of BNBT6.5 were highly affected by trace amount of SMS doping. For composition BNBT6.5-0.003SMS, where T_F-R was near room temperature, a large electric-field-induced unipolar strain of ~0.4% with high normalized strain (S_max/E_max) of 728 pm/V, which is comparable to lead-based ferroelectric/antiferroelectric ceramics, was achieved owing to the reversible electric-field-induced phase transition between a non-polar relaxor phase to a polar phase with long-range ferroelectric order.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

Structure and electrical properties of Ca2+-doped (Na0.47Bi0.47Ba0.06)TiO3 lead-free piezoelectric ceramics

The lead-free piezoelectric ceramics (Na_.47Bi_.47Ba_.06)_1-xCa_xTiO₃ (x?=?0, 0.01, 0.02, 0.03, 0.05, and 0.08, abbreviated as BNBTC/0, BNBTC/1, BNBTC/2, BNBTC/3, BNBTC/5, and BNBTC/8, respectively) were obtained using the solid-state reaction method. The structure, electric conductivity, and dielectric, ferroelectric, and piezoelectric properties of the Ca²⁺-doped (Na_.47Bi_.47Ba_.06)TiO₃ ceramics were thoroughly investigated. The ceramics sintered at 1200?°C exhibit dense microstructures, having relative densities higher than 96%. The X-ray diffraction results demonstrate that all ceramics have a pure perovskite structure. The mean grain sizes of the ceramics are related to the Ca²⁺ quantity. A small quantity of Ca²⁺ ions (x?≤?0.03) improves the piezoelectric and ferroelectric properties of the samples. The dielectric behavior of the samples is sensitive to the Ca²⁺ content and electric poling. The results demonstrate that the electrical properties of the (Na_.47Bi_.47Ba_.06)TiO₃ lead-free ceramics can be well tuned by varying the Ca²⁺ quantity.     

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.     

Temperature‐Dependent Phase Transitions in the Lead‐Free Piezoceramics (1 – x – y)(Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 Observed by in situ Transmission Electron Microscopy and Dielectric Measurements

Lead‐free piezoelectric (1 – x – y)(Bi_1/2Na_1/2)TiO₃–xBaTiO₃–y(K_0.5Na_0.5)NbO₃ (BNT–BT–KNN) ceramics were examined in situ under increasing temperature in the transmission electron microscope. Changing superstructure reflections indicate a transition from rhombohedral to tetragonal to cubic phase with broad coexistence regions. The additional evolution of the microstructure in combination with dielectric measurements leads to a model of two relaxor‐type phase evolutions with temperature.