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.     

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.     

Bi(Zn2/3Nb1/3)O3–(K0.5Na0.5)NbO3 High‐Temperature Lead‐FreeFerroelectric Ceramics with Low Capacitance Variation in a Broad Temperature Usage Range

The xBi(Zn_2/3Nb_1/3)O₃–(1?x)(K_0.5Na_0.5)NbO₃ (abbreviated as xBZN–(1?x)KNN) ceramics have been synthesized using the conventional solid‐state sintering method. The phase structure, dielectric properties and “relaxorlike” behavior of the ceramics were investigated. The 0.03BZN–0.97KNN ceramics show a broad and stable permittivity maximum near 2000 and lower dielectric loss (≤5%) at a broad temperature usage range (100°C–400°C) and the capacitance variation (ΔC/C_150°C) is maintained smaller than ±15%. The 0.03BZN–0.97KNN ceramics only possess the diffuse phase transition and no frequency dispersion of dielectric permittivity, which indicates that 0.03BZN–0.97KNN ceramics is a high temperature “relaxorlike” ferroelectric ceramics. These results indicate that 0.03BZN–0.97KNN ceramics are excellent promising candidates for preparing high‐temperature multilayer ceramics capacitors.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.     

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.     

Dielectric,ferroelectric and field-induced strain behavior of K0.5Na0.5NbO3-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

Lead-free piezoelectric (1 ? x)Bi_0.5(Na_0.78K_0.22)_0.5TiO₃–xK_0.5Na_0.5NbO₃ (BNKT–xKNN, x = 0–0.10) ceramics were synthesized using a conventional, solid-state reaction method. The effect of KNN addition on BNKT ceramics was investigated through X-ray diffraction (XRD), dielectric, ferroelectric and electric field-induced strain characterizations. XRD revealed a pure perovskite phase with tetragonal symmetry in the studied composition range. As the KNN content increased, the depolarization temperature (T_d) as well as maximum dielectric constant (?_m) decreased. The addition of KNN destabilized the ferroelectric order of BNKT ceramics exhibiting a pinched-type hysteresis loop with low remnant polarization (11 μC/cm²) and small piezoelectric constant (27 pC/N) at 3 mol% KNN. As a result, at x = 0.03 a significant enhancement of 0.22% was observed in the electric field-induced strain, which corresponds to a normalized strain (S_max/E_max) of ～434 pm/V. This enhancement is attributed to the coexistence of ferroelectric and non-polar phases at room temperature.     

Dielectric Properties and Impedance Analysis of K0.5Na0.5NbO3–Ba2NaNb5O15 Ceramics with Good Dielectric Temperature Stability

(1?x)(K_0.5Na_0.5)NbO₃–xBa₂NaNb₅O₁₅ [(1?x)KNN–xBNN, 0 ≤ x ≤0.1] ceramics were prepared by solid‐state reaction method. X‐ray diffraction analysis of the ceramics revealed that the crystal structure changed from orthorhombic to rhombohedral with increasing BNN content. Dielectric measurement showed that the ceramics exhibited good dielectric temperature stability over a wide temperature range. Basic mechanisms of the conduction and relaxation processes have been investigated using impedance spectroscopy analyses. It was concluded that the conduction and relaxation processes were thermally activated and oxygen vacancies were the possible ionic charge carriers at higher temperatures.     

Large Strain Response in 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 Lead‐Free Ceramics Induced by the Change of K/Na Ratio in (KxNa1−x)NbO3

Influence of K/Na ratio in (K_xNa_1?x)NbO₃ on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi_0.5Na_0.4K_0.1)TiO₃–0.01(K_xNa_1?x)NbO₃ (BNKT–K_xNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F?R to room temperature. Accordingly, giant strain of ~0.46% (corresponding to a large signal d₃₃* of ~575 pm/V) which is comparable to that of Pb‐based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–K_xNN system and the tolerance factor t of the end number (K_xNN). In situ high‐energy X‐ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field‐induced distortion from the pseudocubic structure.     

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.     

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.     

High‐Temperature Multilayer Ceramic Capacitors Based on 100−x(94Bi1/2Na1/2TiO3–6BaTiO3)–xK0.5Na0.5NbO3

The potential high‐temperature dielectric materials 100?x(94Bi_1/2Na_1/2TiO₃–6BaTiO₃)–xK_0.5Na_0.5NbO₃ with x = 12, 18, and 24 were processed as bulk samples in order to examine the reduction of sintering temperature by means of CuO as sintering aid. Due to the successful reduction of sintering temperature, low cost Ag:Pd could be used as a co‐fired electrode material for multilayer ceramic capacitors (MLCCs). Fabrication of 8 μm thick, dense MLCCs with self‐contained, nonreactive electrodes is reported for a wide range of compositions of Bi_1/2Na_1/2TiO₃–BaTiO₃–K_0.5Na_0.5NbO₃. Among the manufactured MLCCs, those with compositions x = 24 showed the most promising dielectric properties for applications where high operating temperatures are needed. The temperature‐dependence of permittivity was quite low, revealing a change of less than ±10% compared to its 150°C‐value in the range of 40°C–225°C. For samples sintered at 1000°C, an RC constant of about 300 s was obtained at 150°C. Furthermore, these x = 24 MLCCs exhibited the finest microstructures among the compositions examined; making it a suitable candidate for further miniaturization of layer thickness as required for state‐of‐the art devices.     

Strong Red Emissions in Pr3+‐Doped (K0.5Na0.5)NbO3–CaTiO3 Diphase Ceramics

The photoluminescence and temperature sensing properties based on down‐shifting emission of Pr³⁺‐doped (K_0.5Na_0.5)NbO₃–yCaTiO₃ (KNN: yCT) diphasic materials were systematically investigated. Under 447‐nm excitation, Pr³⁺‐doped KNN: yCT samples exhibited significantly enhanced red emission at 603 nm assigned to ¹D₂→³H₄ transitions of Pr³⁺ ions. The red emission intensities reached the optimum value with y = 0.05 near the polymorphic phase transition region. The origin of the enhanced red emission is mainly ascribed to the doping‐induced lattice symmetry change. The energy level transitions from the typical f–f transitions to the valence‐to‐conduction transitions were observed as CaTiO₃ concentration increases above a critical concentration of y = 0.05. Furthermore, the sample with y = 0.05 also possessed excellent temperature response properties in a wide temperature range 300–473 K and the maximum sensing sensitivity was 0.016 K^?1. The Pr³⁺‐doped (K_0.5Na_0.5)NbO₃–yCaTiO₃ red emission materials with admirable intrinsic piezoelectric properties may have important technological promise in novel multifunctional devices.     

Large Electrostrictive Strain in (Bi0.5Na0.5)TiO3–BaTiO3–(Sr0.7Bi0.2)TiO3 Solid Solutions

Relaxor ferroelectrics (0.94 ? x)(Bi_0.5Na_0.5)TiO₃–0.06BaTiO_3?x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT) (0 ≤ x ≤ 0.5), were prepared by a solid‐state reaction process, and their structures were characterized by the transmission electron microscopy and Raman spectroscopy. The BNT–BT–0.3SBT has a very high electrostrictive strain S = 0.152% with hysteresis‐free behavior, much more than the reported S in other ferroelectrics. S–P² profiles perfectly follow the quadratic relation, which indicates a purely electrostrictive effect with a high electrostrictive coefficient (Q₁₁) of 0.0297 m⁴/C². Even, its Q₁₁ keeps at a high level in the temperature range from ambient temperature to 180°C. The field‐induced large electrostrictive strain of BNT–BT–0.3SBT was attributed to the existence of ferroelectric nanodomains.     

Room-temperature ferromagnetism in (K0.5Na0.5)NbO3-xBaNi0.5Nb0.5O3-δ ferroelectric ceramics with narrow bandgap

In this paper, a simple, reproducible and cost-effective solid-state reaction sintering process is developed to fabricate (K_0.5Na_0.5)NbO₃-xBaNi_0.5Nb_0.5O_3-δ (KNN-xBNN) ceramics with a narrow bandgap and room-temperature ferromagnetism. Here, we report a systematic investigation of the influence of the BaNi_0.5Nb_0.5O_3-δ (BNN) concentration on the properties of KNN-xBNN ceramics. All ceramics form orthorhombic perovskite structures with a space group Amm2 and a weak peak at the wavelength of 550 cm^?1 that is characteristic of the pillow shoulder of the orthorhombic phase. KNN-xBNN ceramics with x between 0.02 and 0.08 have a narrow bandgap of about 2.5 eV—much smaller than the 3.5 eV of its parent (K_0.5Na_0.5)NbO₃ (KNN) ceramic—which is attributed to Ni²⁺-oxygen vacancy combinations (Ni²⁺-V_O) raising the valence electron energy level of the KNN ceramic. Furthermore, doping BNN into KNN ceramics can significantly convert the magnetism from diamagnetism to ferromagnetism and the component of x = 0.08 achieves both maximum saturation magnetisation intensity (14 memu/g) and minimum coercive magnetic field (80 Oe). Our findings provide a systematic insight into the bandgap tunability and ferromagnetism induction at room temperature in lead-free perovskite KNN-xBNN ceramics, as well as demonstrate their potential applications in perovskite solar cells and multiferroic devices.     

Optical and Microwave Dielectric Properties of Phase Pure (K0.5Na0.5)NbO3 Thin Films Deposited by RF Magnetron Sputtering

(K_0.5Na_0.5)NbO₃ (KNN) thin films have been deposited onto Pt/Ti/SiO₂/Si and quartz substrates by RF magnetron sputtering. The films were deposited at 400°C with the variation in oxygen mixing percentage (OMP) ratio from 0% to 100% and annealed at 700°C in oxygen atmosphere. The crystallinity of the films is found to be increased with increased OMP. Dielectric properties of the films were examined over the frequency range from 1 kHz to 1 MHz and the temperature range of 30°C to 400°C. The Curie temperature of the films was found to be in the range 369°C–373°C. For the first time, the split postdielectric resonator (SPDR) method was used to measure the microwave (10–20 GHz) dielectric properties of KNN thin films. The optical properties of as‐deposited and annealed KNN thin films were investigated by means of transmittance spectra. The optical bandgap is calculated by using the Tauc relation, and found to be in the range 4.34–4.40 eV and 4.29–4.37 eV for the as‐deposited and annealed films, respectively. The refractive index (n_700nm) of the films found to be in the range 1.98–2.01 and 1.99–2.07 for as‐deposited and annealed films, respectively. The refractive index dispersion is analyzed by using Wemple–DiDomenico (W–D) single‐oscillator model. The effect of annealing and OMP on the refractive index, packing density and W–D parameters has been investigated. The average single oscillator energy (E_o) and dispersion energies (E_d) of the annealed KNN thin films are in the range of 6.17–7.16 eV and 18.77–22.19 eV, respectively. AC‐conductivity of the annealed films was analyzed by using double power law. Ag/KNN/Pt thin films followed the ohmic conduction (J ∝ E^α, where α ~1) and the low leakage current density obtained for the deposited at 100% O₂ is 3.14 × 10^?5 A/cm² at 50 kV/cm.     

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.     

Large Strain Response and Fatigue‐Resistant Behavior in Ternary Bi0.5Na0.5TiO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 Solid Solutions

A ternary solid solution (1 ? x)(0.88Bi_0.5Na_0.5TiO₃–0.12BaTiO₃)‐xBi(Zn_0.5Ti_0.5)O₃ (BNBZT, BNBZTx) was designed and fabricated using the traditional solid‐state reaction method. The temperature and composition dependence of dielectric, ferroelectric, piezoelectric, and fatigue properties were systematically investigated and a schematic phase diagram was proposed. The substitution with Bi(Zn_0.5Ti_0.5)O₃ was found to shift the phase transition (ferroelectric tetragonal to relaxor pseudocubic phase) to lower temperatures. At a critical composition x of 0.05, large electric‐field‐induced strain response with normalized strain S_max/E_max as high as 526 pm/V was obtained under a moderate field of 4 kV/mm around room temperature. The strain exhibited good temperature stability within the temperature range of 25°C–120°C. In addition, excellent fatigue‐resistant behavior was observed in the proposed BNBZT solid solution after 10⁶ bipolar cycles. These give the BNBZT system great potential as environmental friendly solid‐state actuator.     

Excellent Transmittance Induced Phase Transition and Grain Size Modulation in Lead‐Free (1–x)(K0.5Na0.5)NbO3–xLaBiO3 Ceramics

A novel lead‐free excellent transmittance electro‐optic ceramics (1–x)(K_0.5Na_0.5)NbO₃–xLaBiO₃ (KNN‐LB, x = 0.000, 0.005, 0.010, 0.015, 0.020, 0.025, 0.040, 0.060) were fabricated by traditional pressureless ceramics processing procedure. The effects of LaBiO₃ dopant concentration x on the microstructure, phase transition, optical property, and electrical properties were studied systematically. The X‐ray diffraction results indicated that the KNN‐LB ceramics with x ≥ 0.025 have the pseudocubic phase. The morphology, density, and microstructure of the KNN‐LB ceramics were characterized by scanning electronic microscopy and optical microscopy. In particular, the KNN‐LB ceramics (0.05 mm thickness) with x = 0.025 exhibited the highest transmittance of 74.00% in the visible spectrum comparable to the 72.00% transmittance of the lead lanthanum zirconate titanate (PLZT 9/65/35 of 0.127 mm thickness). In addition, the related mechanism of transparency variation induced by phase transition and grain size modulation were discussed thoroughly. Finally, the dielectric and ferroelectric properties of as‐prepared KNN‐LB ceramics were also investigated to further clarify the relationship between transparency and relaxor behavior.     

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.     

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.