Nonstoichiometric effect on dielectric and large-signal electromechanical properties of environmentally friendly BNT-6BT ferroelectric ceramics

Although the nonstoichiometric influence on the small-signal dielectric and piezoelectric properties of (Bi_0.5Na_0.5)TiO₃ (BNT)-based ferroelectrics has been studied extensively over the past decade, the features of large-signal electric field-induced strain (electrostrain), which are of particular importance to actuator devices, have not been thoroughly investigated. In this study, we used the solid-state reaction method to manufacture nonstoichiometric 0.94(Bi_0.5+xNa_0.5?x)TiO₃-0.06BT (BNTx-6BT) ceramics, where x = 0.0–0.05, and investigated the nonstoichiometric effect on the dielectric and large-signal electromechanical properties, with special emphasis on the electrostrain properties. Our results suggest that the room-temperature phase structures of BNTx-6BT ceramics changed from a regular ferroelectric phase to a relaxor ferroelectric phase as the Bi/Na ratio increased from stoichiometric 50/50 to nonstoichiometric 55/45 owing to the nonstoichiometric effect on the long-range ferroelectric order. In the x = 0.02 nonstoichiometric composition, an ultrahigh and electrostrictive-type electrostrain of 0.53% was identified. Compared to their stoichiometric counterparts, nonstoichiometric compositions have stronger temperature stability during polarization, resulting in good temperature stability of the electrostrain. Our findings not only reveal the nonstoichiometric effect on the phase evolution and its impact on the dielectric and large-signal electromechanical properties of BNTx-6BT ceramics but also provide a new method for tailoring the large-signal electrostrain properties of BNT-based ceramics.     

The electrical properties of (1−x)(Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3)–xCaZrO3 lead-free piezoelectric ceramics

Lead-free (1−x)(0.0852Bi_0.5Na_0.5TiO₃–0.12Bi_0.5K_0.5TiO₃–0.028BaTiO₃)–xCaZrO₃ piezoelectric ceramics (BNT−BKT−BT−xCZ, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were prepared by using a conventional solid-state reaction method. The effects of CZ-doping on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT−BKT−BT−xCZ system were systematically investigated. The polarization and strain behaviors indicated that the long-range ferroelectric order in the unmodified BNT−BKT−BT ceramics was disrupted by the increase of CZ-doping content, and correspondingly the depolarization temperature (T_d) shifted down from 109 °C to below room temperature. When x>0.03, accompanied with the drastic decrease in the remnant polarization (P_r) and piezoelectric coefficient (d₃₃), the electric-field-induced strain was enhanced significantly. A large unipolar strain of 0.35% under an applied electric field of 70 kV/cm (S_max/E_max=500 pm/V) was obtained in the BNT−BKT−BT−0.04CZ ceramics at room temperature, which was attributed to the reversible electric-field-induced phase transition between the relaxor and ferroelectric phases.     

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.     

High permittivity and excellent high-temperature energy storage properties of X9R BaTiO3–(Bi0.5Na0.5)TiO3 ceramics

We fabricated x(Bi_0.5Na_0.5)TiO₃–(1−x)[BaTiO₃–(Bi_0.5Na_0.5)TiO₃–Nb] (BNT-doped BTBNT-Nb) dielectric materials with high permittivity and excellent high-temperature energy storage properties. The initial powder of Nb-modified BTBNT was first calcined and then modified with different stoichiometric ratios of (Bi_0.5Na_0.5)TiO₃ (BNT). Variable-temperature X-ray diffraction (XRD) results showed that the ceramics with a small amount of BNT doping consisted of coexisting tetragonal and pseudocubic phases, which transformed into the pseudocubic phase as the test temperature increased. The results of transmission electron microscopy (TEM) showed that the ceramic grain was the core-shell structure. The permittivity of the 5 mol% BNT-doped BTBNT-Nb ceramic reached up to 2343, meeting the X9R specification. The discharge energy densities of all samples were 1.70-1.91 J/cm³ at room temperature. The discharge energy densities of all samples fluctuated by only ±5% over the wide temperature range from 25°C to 175°C and ±8% from 25°C to 200°C. The discharge energy density of the 50 mol% BNT-doped BTBNT-Nb ceramic was 2.01 J/cm³ at 210 kV/cm and 175°C. The maximum energy efficiencies of all ceramics were up to ~91% at high temperatures and were much better than those at room temperature. The stable dielectric properties within a wide temperature window and excellent high-temperature energy storage properties of this BNT-doped BTBNT-Nb system make it promising to provide candidate materials for multilayer ceramic capacitor applications.     

Structural,dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Zn0.5Ti0.5)O3 thin films prepared by sol–gel method

Lead free (1−x)(0.8Bi_0.5Na_0.5Ti_0.5O₃–0.2Bi_0.5K_0.5TiO₃)–xBiZn_0.5Ti_0.5O₃ (x=0–0.06) (BNT–BKT–BZT) thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel processing technique. The effects of BZT content on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT–BKT–BZT thin films were investigated systematically. The BNT–BKT–BZT thin films undergo a transition from ferroelectric to relaxor phase with increasing temperature. The phase transition temperature decreases with the increase of BZT content. The BNT–BKT–BZT thin film with x=0.04 exhibits the best ferroelectric properties (P_max=40 µC/cm² and P_r=10 µC/cm²), largest dielectric constant (ε=560) and piezoelectric constant (d₃₃=40 pm/V). This finding demonstrates that the BNT–BKT–BZT thin film has an excellent potential for demanding high piezoelectric properties in lead free films.     

Phase structure,ferroelectric properties,and electric field-induced large strain in lead-free 0.99[(1−x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3]–0.01Ta piezoelectric ceramics

Lead-free 0.99[(1−x)(Bi_0.5Na_0.5)TiO₃-x(Bi_0.5K_0.5)TiO₃]–0.01Ta piezoelectric ceramics were prepared by a conventional solid-state reaction process. The ferroelectric properties, and strain behaviors were characterized. Increase of the (Bi_0.5K_0.5)TiO₃ content induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic phase. Accordingly, the ferroelectric order is disrupted significantly with the increase of (Bi_0.5K_0.5)TiO₃ content and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of 0.35% (corresponding to a large signal d₃₃ of 438 pm/V) in samples with 20 mol% (Bi_0.5K_0.5)TiO₃ content. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced nonpolar pseudocubic-to-polar ferroelectric phase transformation.     

Ultra‐Wide Temperature Stable Dielectrics Based on Bi0.5Na0.5TiO3–NaNbO3 System

The microstructure, phase structure, ferroelectric, and dielectric properties of (1?x)Bi_0.5Na_0.5TiO₃‐xNaNbO₃ [(1?x)BNT‐xNN] ceramics conventionally sintered in the temperature range of 1080°C–1120°C were investigated as a candidate for capacitor dielectrics with wide temperature stability. Perovskite phase with no secondary impurity was observed by XRD measurement. With increasing NN content, (1?x)BNT‐xNN was found to gradually transform from ferroelectric (x = 0–0.05) to relaxor (x = 0.10–0.20) and then to paraelectric state (x = 0.25–0.35) at room temperature, indicated by P–I–E loops analysis, associated with greatly enhanced dielectric temperature stability. For the samples with x = 0.25–0.35, the temperature coefficient of capacitance (TCC) was found <11% in an ultra‐wide temperature range of ?60°C–400°C with moderate dielectric constant and low dielectric loss, promising for temperature stable capacitor applications.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Al0.5Ga0.5)O3 Lead‐Free Piezoceramics

A new lead‐free BNT‐based piezoelectric ceramics of (1 ? x)Bi_0.5Na_0.5TiO₃–xBi(Al_0.5Ga_0.5)O₃ (x = 0, 0.02, 0.03, 0.04, and 0.05) were synthesized using a conventional ceramic fabrication method. Their structures and electrical properties were investigated. All the samples show a typical ferroelectric P(E) loops and S(E) curves at room temperature. The optimal properties are obtained at the composition of the x = 0.03. The substitution of Bi(Al_0.5Ga_0.5)O₃ enhances piezoelectric constant and increases Curie temperature from 58 pC/N and 310°C of pure BNT to 93 pC/N and 325°C of the x = 0.03. The temperature‐dependent P(E) loops and S(E) curves of 0.97BNT–0.03BAG indicate that phase transition from ferroelectric to antiferroelectric takes place over a very wide temperature region from 80°C to 180°C. The results show that the introduction of BAG improves the electrical properties of BNT.     

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 dielectric,piezoelectric properties and strengthened relaxor behavior in K-modified Na0·5Bi0·5TiO3 lead-free ceramics

The effect of K⁺ modification on the phase composition, micro structure and electrical properties of Na_0·5Bi_0·5TiO₃ ceramics prepared via solid phase reaction was investigated. The samples (Na_1-xK_x)_0.5Bi_0·5TiO₃ still present pure perovskite structure. The doping K⁺ can increase grain size and relative density, and improve the electrical properties of BNT samples. The dielectric constant of (Na_1-xK_x)_0.5Bi_0·5TiO₃ samples increased from 800 to 1844, the piezoelectric constant from 61 pC/N to 172 pC/N, and the dielectric constant at Curie temperature increased from 1950 to 7182. The samples modified by K⁺ exhibit strong relaxation performance and excellent dielectric-temperature stability at high temperatures. The mechanism of K⁺ modification was explained and analyzed in detail using domain theory, lattice theory and ferroelectric relaxation theory.     

Enhanced microstructure and electrical properties of Mn-modified Bi0.5(Na0.65K0.35)0.5TiO3 ferroelectric ceramics

Bi_0.5(Na_0.65K_0.35)_0.5TiO₃ (BNKT) and Mn-modified Bi_0.5(Na_0.65K_0.35)_0.5(Mn_xTi_1−x)O₃ (BNKMT-10³x), (x=0.0–0.5%) ferroelectric ceramics were synthesized by solid-state reaction method. Optimization of calcination temperature in Mn-doped ceramics was carried out for the removal of secondary phases observed in XRD analysis. BNKMT ceramics sintered at 1090 °C showed enhanced dielectric, piezoelectric and ferroelectric properties in comparison to pure BNKT. The average grain size was found to increase from 0.35 μm in BNKT to 0.52 μm in Bi_0.5(Na_0.65K_0.35)_0.5(Mn_0.0025Ti_0.9975)O₃ (BNKMT-2.5) ceramics. The dielectric permittivity maximum temperature (T_m) was increased to a maximum of 345 °C with Mn-modification. AC conductivity analysis was performed as a function of temperature and frequency to investigate the conduction behavior and determine activation energies. Significant high value of piezoelectric charge coefficient (d₃₃=176 pC/N) was achieved in BNKMT 2.5 ceramics. Improved temperature stability of ferroelectric behavior was observed in the temperature dependent P–E hysteresis loops as a result of Mn-incorporation. The fatigue free nature along with enhanced dielectric and ferroelectric properties make BNKMT-2.5 ceramic a promising candidate for replacing lead based ceramics in device applications.     

Structural,dielectric and ferroelectric properties of Ba1−x(Bi0.5Na0.5)xTiO3 ceramics

Lead free Ba_1?x(Bi_0.5Na_0.5)_xTiO₃ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) ferroelectric ceramics were synthesized by conventional solid state reaction technique. Sintering was done at 1200 °C for 2 h in air atmosphere. The final products have tetragonal symmetry with decreasing c/a ratio confirmed by X-ray diffraction analysis. The grain size varies between 300 nm to 1000 nm for x=0 to 0.1. With increase in Bi_0.5Na_0.5TiO₃ [BNT] content, the room temperature permittivity decreases whereas the Curie temperature (T_c) increases and its highest value was found to be 155 °C for 10 mol% of BNT addition. The ceramics show stable and low dielectric loss characteristics. The remnant polarization (P_r) and the coercive field (E_c) increases monotonously with increase in BNT content. The highest value of 2P_r (=17 μC/cm²) and 2E_c (=22 Kv/cm) was obtained for x=10 mol% BNT addition.     

Dielectric temperature stability of Nb-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

In this study, the phase structure, microstructure and dielectric properties of Bi_0.5(Na_0.78K_0.22)_0.5(Ti_1-xNb_x)O₃ lead-free ceramics prepared by traditional solid phase sintering method were studied. The second phase pyrochlore bismuth titanate (Bi₂Ti₂O₇) was produced in the system after introduction of Nb⁵⁺. The dielectric constant of the sample (x = 0.03) sintered at 1130 °C at room temperature reached a maximum of 1841, and the dielectric loss was 0.045 minimum. It had been found that the K⁺ and Nb⁵⁺ co-doped Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics exhibited outstanding dielectric-temperature stability within 100–400 °C with T_cc ≤±15%. Result of this research provides a valuable reference for application of BNT based capacitors in high temperature field.     

Large electrostrain at high temperature in bismuth sodium titanate-based piezoelectric ceramics

(1-x)[0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃]-xAgNbO₃ ceramics (abbreviated as BNKT-100xAN, x = 0–0.06) are fabricated by the solid state reaction method. It is discovered that silver niobate is completely dissolved into the lattice of 0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃ ceramics based on characterization. The relaxation mechanism and characteristics of BNKT-100xAN ceramics are analyzed by dielectric temperature spectra and AC impedance spectra. The content of ordered domains and PNRs of BNKT-4AN ceramics is optimum, which enables it possess excellent electrostrain of 0.51%. The electrostrain almost remain constant after the 10⁴ cycles test, suggesting that it also has great fatigue resistance. After polarization at high temperature, large electrostrain of 1.4% and inverse piezoelectric constants of 2004 pm/V are obtained at 150 °C in BNKT-4AN ceramics. The ultrahigh electrostrain is mainly promoted by the larger polarizability of the <111>-oriented and <001>-oriented defect dipoles, the same direction of defect dipoles and the spontaneous polarization direction of phase, the pinning effects of the defect dipoles on the PNRs and the collinear arrangement of the PNRs. This work indicates that lead-free piezoelectric ceramics is hopeful to be applied in piezoelectrics working at high temperature. Simultaneously, a new design paradigm also is provided for the preparation of piezoelectric ceramics with excellent performance.     

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.     

Strain properties of (1-x)Bi0.5Na0.4K0.1TiO3-xBi(Mg2/3Ta1/3)O3 electroceramics

A ternary solid solution of Bi_0.5Na_0.5TiO₃–Bi_0.5K_0.5TiO₃–Bi(Mg_2/3Ta_1/3)O₃ (BNKT-xBMT) lead-free electroceramics was synthesized by a solid-state reactive sintering technique. The electrostrain, dielectric, and ferroelectric properties as well as the impedance characteristics and the microstructure were systematically assessed. With the increase of BMT, the BNKT-xBMT ceramics gradually transformed from non-ergodic relaxor phase to ergodic relaxor phase, manifested as the ferroelectric-to-relaxor temperature (T_F-R) shifts towards below room temperature. Additionally, the ferroelectric hysteresis curves became pinched, and the strain curve changed from butterfly-shaped into sprout-shaped. At the ergodic relaxor composition of x = 0.04, a large electrostrain value (S = 0.4%; under an electric field of 60 kV/cm, d₃₃* = 632 pm/V) was achieved, which is mainly attributed to the electric-field-induced transition from the ergodic relaxor phase to the ferroelectric phase.     

Structural evolution and ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-xSrTiO3 solid solutions

Bi_0.5Na_0.5TiO₃ (BNT)-based dielectric ceramics have received a lot of attention due to the increased demand for pulse ceramic capacitors. However, comprehensive study on the relationship between their internal phase structure, dielectric characteristics, and ferroelectric properties is still lacking. The phase evolution and its impact on dielectric and ferroelectric properties of an important BNT-based solid solution, Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ (x = 0, 0.1, 0.2, 0.3 and 0.4), were investigated systematically in this work using structural, dielectric, and ferroelectric characterization techniques. X-ray diffraction indicated the coexistence of rhombohedral and tetragonal phases. The frequency- and temperature-dielectric characterization was then used to derive the characteristic temperatures T_B, T_m, T_s, and T_d, and a phase diagram was developed. Furthermore, the temperature-dependent current against electric field curves and polarization versus electric field loops were used to derive the characteristic temperatures connected to high electric field features. This study not only explains the phase evolution of the Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ solid solution, but it also correlates microscopic domains and polar nanoregions to macroscopic dielectric and ferroelectric properties.     

Phase development and dielectric responses in PMN–BNT ceramics

(1 − x)Pb(Mg_1/3Nb_2/3)O₃–x(Bi_0.5Na_0.5)TiO₃ ceramics were prepared by the conventional mixed-oxide method. All compositions show complete perovskite solid solutions and the structure to change from cubic to rhombohedral at x = 0.5. The dielectric constant and dielectric loss tangent were measured as a function of both temperature and frequency. The results indicated a relaxor ferroelectric behavior for all ceramics. The temperature at maximum of the dielectric constant of PMN–BNT ceramics were seen to increase with increasing BNT content. Moreover, the broadest dielectric peak occurs at x = 0.9, which leads to a morphotropic phase boundary in this system.     

Tailoring structural and electrical properties of A-site non-stoichiometric Na0.5Bi0.5TiO3 ceramic at different sintering temperature

Na/Bi stoichiometry plays crucial role in determining various properties of sodium bismuth titanate-based system. In this work, we have synthesised lead free (Na_0.5Bi_0.5)_1+x TiO₃ (x?=?0, 0.02 and 0.05) ceramics by sol-gel method and systematically presented structural, dielectric and ferroelectric properties at different sintering temperature. Single phase perovskite structure with rhombohedral symmetry (R3c) is obtained for all compositions from low (850°C) to maximum (1150°C) sintering temperature. The shifting of x-ray diffraction peaks and characteristic perovskite metal-oxide vibrational band (～627?cm^?1) in Fourier Transform Infra-red spectra suggests compression or expansion of crystal lattice with Na/Bi non-stoichiometry. Excess of Na/Bi comprises dense crystal growth as compared to pure Na_0.5Bi_0.5TiO₃ composition suggesting compensation of volatile elements loss during heat treatment whose impact has also been observed in dielectric as well as ferroelectric properties. It is observed that Na_0.51Bi_0.51TiO₃ sample with x?=?0.02 exhibits better structural, dielectric and ferroelectric properties in whole range of sintering temperature.     

Phase transition and energy storage properties of Bi0.5Na0.5TiO3–Bi(Mg2/3Nb1/3)O3 lead-free ceramics

Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics have been extensively studied due to their excellent dielectric, piezoelectric and ferroelectric properties. The phase structure and functionalities of BNT can be feasibly adjusted by doping/forming solid solutions with other elements/components. In this work, Bi(Mg_2/3Nb_1/3)O₃ (BMN) was introduced into BNT by a conventional solid-state reaction to form a homogeneous solid solution of (1-x)(Bi_0.5Na_0.5)TiO₃-xBi(Mg_2/3Nb_1/3)O₃ (BNT-xBMN) with a perovskite structure. With the increase of BMN content, a phase transition from rhombohedral R3c to tetragonal P4bm has been confirmed by XRD, along with shifting the ferroelectric-paraelectric phase transition temperature to lower temperatures with broadening dielectric peaks. Furthermore, an optimized recoverable energy density of 1.405 J/cm³ was achieved for BNT-0.10BMN ceramics under a low applied electric field of 140 kV/cm, which is mainly attributed to the transformation from ferroelectric to ergodic relaxor phase.