Thermally-stable large strain in Bi(Mn0.5Ti0.5)O3 modified 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics

(1-x)[0.8Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃]-xBi(Mn_0.5Ti_0.5)O₃ (x = 0–0.06, BNKMT100x) lead-free ferroelectric ceramics were prepared via solid state reaction method. Bi(Mn_0.5Ti_0.5)O₃ induces a structure transition from rhombohedral-tetragonal morphotropic phases to pseudo-cubic phase. Moreover, the wide range of compositions within x = 0.03–0.055 exhibit large strain of 0.31%–0.41% and electrostrictive coefficient of 0.027–0.041 m⁴/C². Especially, at x = 0.04, the large strain and electrostrictive coefficient are nearly temperature-independent in the range of 25–100 °C. The impedance analysis shows the large strain and electrostrictive coefficient originate from polar nanoregions response due to the addition of Bi(Mn_0.5Ti_0.5)O₃.     

Orientation dependence of electric field induced phase transitions in lead-free (Na0.5Bi0.5)TiO3-based single crystals

The orientation dependence of the electric field induced strain and phase transitions in 0.92(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃–0.02(K_0.5Na_0.5)NbO₃ (NBT–6BT–2KNN) single crystals has been investigated. The evolution of Raman spectra with electric field reveals that a tetragonal ferroelectric phase is initially induced at E = 14 kV/cm and completed above E = 25 kV/cm for [001] oriented single crystals. When the electric field is applied along [111] direction, a partial phase transition from pseudocubic to rhombohedral structure is triggered at E = 19 kV/cm, which is higher than that for inducing tetragonal ferroelectric phase along [001] direction. Both field-induced phase structures and stability of NBT–6BT–2KNN single crystal are strongly associated with the crystallographic orientations. These results provide a better understanding to the field-induced macroscopic strain in lead-free NBT-based ferroelectrics.     

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 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.     

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.     

Normal-relaxor ferroelectric phase transition induced morphotropic phase boundary accompanied by enhanced piezoelectric and electrostrain properties in strontium modulated Bi0.5K0.5TiO3 lead-free ceramics

In this work, (Bi_0.5K_0.5)_1-xSr_xTiO₃ compositions (x = 0.03∼0.18) are designed to clarify the role of normal-relaxor ferroelectric phase transition and morphotropic phase boundary on dielectric, piezoelectric and electrostrain properties. With increasing strontium content, tetragonal distortion decreases and tetragonal and pseudocubic phases coexist in 0.09 ≤ x ≤ 0.15 compositions; the spontaneous phase-transition temperature and curie temperature decrease, as certified by phase-structure, dielectric properties and Raman spectra analysis. Optimized piezoelectric constant ∼106 pC/N and electrostrain ∼0.17 % are obtained for (Bi_0.5K_0.5)_0.88Sr_0.12TiO₃ composition. Piezoelectric force microscopic technique is exploited to clarify the origin of enhancement in macroscopic performances. Increase in temperature enhances ferroelectric performance and a large strain value ∼0.25 % with low hysteresis ∼27 % are obtained at 140 °C for the optimized composition, which are believed to originate from electric-field induced relaxor-to-ferroelectric phase transition with thermally-activated reduced energy barriers. This work clearly demonstrates that lead-free Bi_0.5K_0.5TiO₃-based ceramics are another promising bismuth-based species in applications of piezoelectric sensors and actuators.     

Effect of dysprosium doping on structural and vibrational properties of lead-free (Na0.7K0.3)0.5Bi0.5TiO3 ferroelectric ceramics

This study describes the effect of Dy³⁺ doping of lead-free ferroelectric sodium potassium bismuth titanate ((Na_0.7K_0.3)_0.5Bi_0.5TiO₃; NKBT) ceramics on their structural, optical, and vibrational properties. X-ray diffraction and Rietveld refinement analyses revealed that NKBT samples exhibited a rhombohedral perovskite structure belonging to the R3c space group, with the incorporation of Dy³⁺ resulting in structural parameter change and local atomic structure shift to tetragonal P4mm, as supported by the appearance of new Raman modes. Increased Dy³⁺ content decreased the NKBT band gap from 3.4 to 3.3 eV, which was ascribed to the introduction of Dy 4f levels between valence and conduction bands. Ferroelectric behavior studies combined with transport measurements explained the unexpected ferroelectric response weakening of Dy³⁺-modified NKBT by increased leakage current densities.     

Origin of enhanced depolarization temperature in quenched Na0.5Bi0.5TiO3-BaTiO3 ceramics

Na_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-BT)-based lead-free piezoelectric ceramics have been actively studied in recent years as a potential replacement for lead-based materials in ultrasonic applications. However, its relatively low thermal depolarization temperature (T_d) is still an imperative obstacle hindering implementation in practical application. Recently, it was reported that quenching is an effective way to improve T_d of NBT-based ceramics, but the essential mechanism is still unclear. In this study, 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ (NBT-6BT) ceramics were quenched in air and liquid nitrogen, and then annealed in oxygen and nitrogen atmospheres to explore the origin of enhanced depolarization temperature. The results from this study correlate the enhancement of T_d to the residual stress, which induces a stable rhombohedral ferroelectric phase, thereby increasing the thermal depolarization temperature of NBT-6BT. Our results indicate that the residual stress is also an important factor influencing the electrical properties of quenched piezoelectric ceramics, which should be given more attention in future studies.     

Shock-driven depolarization behaviors and electrical output in BiAlO3-doped Bi0.5Na0.5TiO3 ferroelectric ceramics

Ferroelectric materials under shock compression can generate current and power by a drastic change of the remnant polarizations and surface bound charge. This behavior has been employed in applications involving nuclear fusion trigger, energy storage devices, and high pulse power sources. Despite the large power output in lead-containing ferroelectrics, lead-free materials are highly desirable owing to the environmental concerns. Herein, the phase transition behaviors and current outputs of 0.92Bi_0.5Na_0.5TiO₃-0.08BiAlO₃ (BNT-8BA) materials are studied under high pressure. The BNT-8BA ferroelectric ceramics can be completely depolarized from polar to the nonpolar state under shock compression, resulting in a current output in the external circuit. The phase-transition-induced depolarization pressures of BNT-8BA are lower than those of the pure Bi_0.5Na_0.5TiO₃ under both dynamic and static high-pressure loads. These results can allow the understanding of the high-pressure behavior of BNT-8BA for application as ferroelectric pulsed power supply.     

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.     

Influence of K0.5Bi0.5TiO3 on the structure,dielectric and ferroelectric properties of (Ba,Ca)(Zr,Ti)O3 ceramics

A new lead-free ferroelectric solid solution between (Ba,Ca)(Zr,Ti)O₃ (BCZT) and K_0.5Bi_0.5TiO₃ (KBT) has been systematically investigated in terms of its phase transformations, microstructure, dielectric and ferroelectric properties. The incorporation of KBT into BCZT was found to enhance the sintering behavior, although secondary phases of K₄Ti₃O₈ and BaBi₄Ti₄O₁₅ were detected at high KBT contents. Chemical heterogeneity was also observed in the form of core-shell grain structures comprising tetragonal ferroelectric BCZT-rich cores with pseudo-cubic relaxor ferroelectric KBT-rich shell regions. Temperature-dependent dielectric property measurements revealed that the BCZT-KBT ceramics exhibited both normal and relaxor ferroelectric behaviour simultaneously, associated directly with the core-shell structure. Ferroelectric hysteresis measurements indicated that the remanent polarisation and coercive field were strongly dependent on KBT content. In common with other lead-free relaxor ferroelectrics, increasing temperature led to the formation of constricted polarisation-electric field hysteresis loops, indicating the occurrence of a reversible electric field-induced nanopolar to long-range ordered ferroelectric state.     

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.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Lead-free pyroelectric infrared detector based on (Bi1/2Na1/2)TiO3-BaTiO3 ferroelectric ceramics

In this paper, we theoretically and experimentally reported a lead-free pyroelectric infrared (PIR) detector using (Bi_1/2Na_1/2TiO₃)-BaTiO₃(BNT-BT) ferroelectric ceramics as the sensitive material. The variation of noise density, voltage response rate (R_V), and specific detection rate (D*) with the modulation frequency under the current mode amplification circuit was investigated, and it was found that the lead-free PIR detector showed high R_V in the low frequency band. The R_V and D* reached 1.51 × 10⁵ V/W and 2.02 × 10⁸ cmHz^1/2W⁻¹ at 10 Hz, respectively. The results were much superior to the PIR based on traditional commercial pyroelectric ceramics, indicating that BNT-BT lead-free ceramics have great potential in application to PIR detectors.     

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.     

Optimized energy storage performances in morphotropic phase boundary (Na0.8K0.2)0.5Bi0.5TiO3-based lead-free ferroelectric thin films

As microelectronic devices move toward integration and miniaturization, the thin film capacitors with high energy density and charge/discharge efficiency have attracted immense interests in modern electrical energy storage systems. Despite morphotropic phase boundary (Na_0.8K_0.2)_0.5Bi_0.5TiO₃-based lead-free materials with outstanding ferroelectric and piezoelectric properties, while large ferroelectric hysteresis with high remanent polarization (P_r) hinder to improve energy storage capability. Here, novel lead-free relaxor-ferroelectric (RFE) thin film capacitors with high energy density are successfully prepared in (1-x) (Na_0.8K_0.2)_0.5Bi_0.5TiO₃-xBa_0.3Sr_0.7TiO₃ [(1-x)NKBT-xBST] systems. Introducing BST into the NKBT systems is expected to reduce remanent polarization (P_r) on account of coupling reestablishment of the polar nano-regions (PNRs) and improving the relaxation behavior. As a result, 0.6NKBT-0.4BST thin film exhibits high energy density (W_rec ～ 54.79 J/cm³) together with satisfactory efficiency (η ～ 76.42%) at 3846 kV/cm. The stable energy storage performances are achieved within the scope of operating temperatures (20–200 °C) and fatigue cycles (1-10⁷ cycles). This work furnishes a new technological way for the design of high energy-density thin film capacitors.     

Influence of the (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoceramic geometries on the power generation of energy harvesting devices

In this work, we report the harvested energy of lead-free 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ bulk ceramics using two different geometries (rectangular plate and disk). These ceramics were obtained by the solid-state reaction method and their good ferroelectric properties were reported. The samples were characterized using X-ray diffraction (XRD), Raman microspectroscopy and Scanning Electron Microscopy (SEM) analysis, whereas ferroelectric and dielectric properties were evaluated by means of ferroelectric hysteresis loops and impedance spectroscopy studies. A cantilever-based harvester was proposed to evaluate the performance of Piezoelectric Energy Harvesting (PEH). The morphological influence on the voltage generation was experimentally and numerically studied by the Finite Element Method (FEM). Several cases for plate and disk-shaped piezoelectric ceramics were simulated under a constant volume restriction. Results showed a great influence of the morphology on the electromechanical response of the system. In particular, the plate featured a higher voltage generation than the disk.     

The effects of aliovalent cations doping on electric-field-induced strain and microstructures of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free piezoceramics

Effects of Al and Nb doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃(abbreviated as BNB6T) ceramics on dielectric, ferroelectric, and strain properties were investigated. The results indicated that the d₃₃^? value is significantly higher in pure BNB6T ceramics than in Nb doped BNB6T and Al doped BNB6T ceramics, and that S–E curve for the Al doped BNB6T ceramics displayed a typical W-shaped butterfly curve, but pure BNB6T and Nb doped BNB6T ceramics display a V-shaped curve. The P–E loops for pure BNB6T and Nb doped BNB6T ceramics were similar to soft PZT ceramics with slim profiles. According to the results of microstructures analysis, significant strain may originate from the development of the optimum compound domains coexistence (the mixture of microdomain and nanodomain), which provide large saturation polarization and low-energy barrier dipole switching under an applied electric field. All these would provide a new concept for lead-free ceramics to realize high strain response for practical applications in actuator or other piezoelectric devices.     

Optical dispersion and bandgap of pure and Mn-doped 0.92Na0.5Bi0.5TiO3-0.08K0.5Bi0.5TiO3 lead-free single crystals

Optical properties of lead-free ferroelectric pure and Mn-doped 0.92Na_0.5Bi_0.5TiO₃-0.08K_0.5Bi_0.5TiO₃ (NBT-8KBT) single crystals have been investigated systematically. Refractive index and extinction coefficient were measured and the critical parameters are obtained by modified Sellmeier dispersion equations and single-oscillator dispersion relation. The decline of refractive index for Mn:NBT-8KBT could be related to the lattice distortion of the Mn ions doping. High transmittance (>70%) over the transparent region (>400 nm) was found in the pure NBT-8KBT, higher than that of Mn:NBT-8KBT, which could be caused by the increase of domain wall density. An about 100 nm red-shift of absorption edge was observed in the Mn:NBT-8KBT single crystal, which is located in the visible region. Optical bandgap energies calculated through Tauc equation and a large bandgap difference of 2.96 and 2.62 eV occurs in the NBT-8KBT and Mn:NBT-8KBT, respectively, which illustrates the modulation of the band structures by Mn doping.     

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.