Enhanced piezoelectric property in Mn-doped K0.5Na0.5NbO3 ceramics via cold sintering process and KMnO4 solution

Through mixing the KMnO₄ solution with K_0.5Na_0.5NbO₃ (KNN) powders, cold sintering process (CSP) was employed to fabricate high-density Mn-doped KNN green pellets and ceramics. The microstructure, doping effect of Mn and electrical properties of these ceramics were studied in detail. Compared with conventional sintering (CS), the CSP supports the homogeneity of dopants and then promotes grain growth and ceramic densification; thus the Mn-doped KNN ceramics prepared by CSP show the obviously higher density and larger grain size. Besides, the less alkalis volatilization and oxygen vacancies result in more Mn³⁺ but less Mn⁴⁺ in CSP ceramics compared to CS ones, which endows the pinning effect and good poling characteristics in CSP ceramics. All the previous results contribute to the high dielectric constant and remnant polarization in CSP ceramics, which support the enhanced piezoelectric coefficient and are much superior than Mn-doped KNN ceramics prepared by CS. This work reveals that CSP can be a new doping strategy to perform chemical modification of electrical properties in KNN ceramics.     

K0.5Na0.5NbO3 ceramics fabricated by combining cold sintering with annealing in air atmosphere or low pO2 atmosphere

Dense K_0.5Na_0.5NbO₃ lead-free ceramics were successfully fabricated by combining cold sintering process (CSP) with annealing in air atmosphere or low pO₂ atmosphere. Acetic acid was used as transition liquid. By optimizing the liquid content and pressure during CSP, the CSP samples with relative densities 74.2% were obtained with adding 15 wt% acetic acid under 800 MPa. The CSP samples were then annealed in air atmosphere between 1000 and 1100°C. The ceramics annealed at 1050°C exhibit excellent electrical properties with piezoelectric constant d₃₃ = 125 pC/N, dielectric constant ε_r = 509, dielectric loss tan δ = 0.04, and remanent polarization P_r = 38.17 μC/cm². Additionally, the mixture of K_0.5Na_0.5NbO₃ and base metal Ni powders was also cold-sintered and then annealed in the low pO₂ atmospheres in order to detect whether Ni was oxidized or not. The optimized low pO₂ atmospheres to protect the Ni from oxidation are pO₂ = 10⁻¹⁰ atm followed by reoxidizing in pO₂ = 10⁻⁶ atm. The K_0.5Na_0.5NbO₃ fabricated by CSP with annealing in the optimized low pO₂ atmospheres exhibits comparable electrical properties of the ceramics annealed in air atmosphere, providing an optional approach for using base metals as electrodes.     

Phase-composition dependent domain responses in (K0.5Na0.5)NbO3-based piezoceramics

Lead-free (K_0.5Na_0.5)NbO₃-based (KNN) piezoceramics featuring a polymorphic phase boundary (PPB) between the orthorhombic and tetragonal phases at room temperature are reported to possess high piezoelectric properties but with inferior cycling stability, while the ceramics with a single tetragonal phase show improved cycling stability but with lower piezoelectric coefficients. In this work, electric biasing in-situ transmission electron microscopy (TEM) study is conducted on two KNN-based compositions, which are respectively at and off PPB. Our observations reveal the distinctive domain responses in these two ceramics under cyclic fields. The higher domain wall density in the poled KNN at PPB contributes to the high piezoelectric properties. Upon cycling, however, a new microstructure feature, “domain intersection”, is directly observed in this PPB composition. In comparison, the off-PPB KNN ceramic develops large domains during poling, which experience much less extent of disruption during cycling. Our comparative study provides the basis for understanding the relation between phase composition and piezoelectric performance.     

High performance lead-free Na0.5K0.5NbO3 piezoelectric ceramics obtained via oscillatory hot-pressing

Lead-free Na_0.5K_0.5NbO₃ (KNN) piezoelectric ceramics is regarded as a potential candidate for PZT material, while high performance is difficult to be obtained due to its poor sinterability and non-stoichiometric component. In this work, oscillatory pressure-assisted hot pressing (OPAHP) is utilized to fabricate KNN ceramics with high density. The KNN ceramics sintered at 860 °C exhibits superior performance with piezoelectric parameter (d₃₃) of 142 pC/N, electromechanical coupling factors (k_p) of 0.41, and relative permittivity (ε^T₃₃/ε₀) of 472–620. Additionally, hardness and flexural strength are measured as 3.55 GPa and 99.13 MPa, respectively. This work indicates that OPAHP technique is effective for fabricating KNN piezoelectric ceramics with high performance.     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

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.     

Piezoelectric enhancements in K0.5Na0.5NbO3-based ceramics via structural evolutions

The current work aims to compare the effect of systematic A-site and B-site substitutions on the piezoelectricity of Ka_0.5Na_0.5NbO₃ (KNN)-based perovskite ceramics. The A-site elements was replaced by Li⁺ while Nb5⁺ was substituted by Sb⁵⁺ to form (K_0.4675Na_0.4675Li_0.065)NbO₃ (KNLN) and (K_0.4675Na_0.4675Li_0.065)(Nb_0.96Sb_0.04)O₃ (KNLNS) respectively. The ceramics were prepared using solid-state sintering method. The density of the ceramics steadily improved with the substitutions while the crystal structure evolved from monoclinic (in KNN) to the coexistence of monoclinic and tetragonal (in KNLN) and finally tetragonal in KNLNS. Distinct variations on size and morphology were recorded. Although density, crystal structure and morphology have minor effect on the E_c, they imposed considerable influences on P_r, d₃₃ and k_p. Despite relatively lower density, KNLN exhibited the highest P_r, d₃₃ and k_p at 9.80 μC/cm²,185 pC/N and 0.43 respectively signifying the positive enhancement brought by the co-existence of monoclinic and tetragonal crystal structures. More importantly, this work systematically proved that the co-existence of both structures signified the morphotropic phase boundary (MPB) composition as the primary factor for the enhancement of KNN piezoelectric properties.     

Tetragonal tungsten bronze phase potential in increasing the piezoelectricity of sol-gel synthesized (K0.5Na0.5)1-xLixNbO3 ceramics

(K,Na)NbO₃ (KNN)-based ceramics have been proven to be formidable candidates among lead-free piezoelectric materials, yet poor reproducibility always hinders their progress. In the present study, the effects of low lithium substitution on the electrical properties and microstructure of (K_0.5Na_0.5)_1-xLi_xNbO₃ (KNLN) ceramics were investigated. All samples were synthesized by the sol-gel method. The Curie temperature (T_C) of the ceramics shifted to higher temperature and gradually decreased the monoclinic-tetragonal (T_M-T) phase transition. Li⁺ substitution had a prominent effect on the ferroelectric properties and improved the piezoelectric coefficient (d₃₃) up to 181 pC/N. X-Ray Diffraction (XRD) studies and Field Emission Scanning Electron Microscopy (FESEM) images revealed an inevitable tetragonal tungsten bronze (TTB) secondary phase, which was formed during the preparation process. It was demonstrated that the volatilization of Li⁺ cations facilitated TTB growth. The coexistence of two different phase structures proved to enhance the KNN piezoelectric performance.     

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.     

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO₃-0.5NaNbO₃ (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ～92 to 96 % theoretical density. Typically, sintering temperatures between ～1000?1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.     

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

In recent years, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) has become popular and promising among perovskite lead‐free piezoceramic systems. In this study, the chemical stability of KNN powders in aqueous medium was investigated as a function of pH, time, and powder surface area. To better understand the dissolution behavior of the complex KNN stoichiometry, subconstituents such as potassium niobate (KNbO₃, KN) and sodium niobate (NaNbO₃, NN) were investigated separately first. Results showed that all of the cations in the structure underwent dissolution in different values. Indicating that KNN undergoes incongruent dissolution in aqueous medium, the dissolution of A site cations was higher at lower pH while the dissolution of B site cations increased at higher initial pH. The order of released cation concentrations (C_A1 = K > C_{A2 = Na} > C_B = Nb) fits with inverse relationship of cation field strength (FS) order, B = Nb⁵⁺_FS > A₂ = Na⁺_FS>A₁ = K⁺_FS, at pH 4, 7 and 10 for NN, KN, and KNN. Calculated diffuse layer thickness from the ICP data confirmed to outer amorphous layer in TEM image. Also, the ratio of normalized cation concentration versus surface area of powders showed that incongruent dissolution kinetic was driven by the diffusion step.     

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.     

Improved Li and Sb doped lead-free (Na,K)NbO3 piezoelectric ceramics for energy harvesting applications

Piezoelectric energy harvesting is the most widely investigated technology for renewable energy applications. In this work, (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ piezoelectric ceramics were prepared through conventional mixed oxide fabrication methods with different sintering temperatures. Although the (Na_0.5K_0.5)NbO₃ piezoelectric material is representative among the lead-free ceramics, it is difficult to densify by typical sintering techniques owing to its easy evaporation properties of potassium (K⁺) and sodium ion (Na⁺). Hence, lithium (Li⁺) and antimony ion (Sb⁵⁺) were used for the partial substitution of (Na_0.5K_0.5)NbO₃. With the optimized sintering temperature, Li⁺ and Sb⁵⁺ are expected to be crucial in increasing the density and enhance the piezoelectric and ferroelectric properties. In this study, the phase, microstructure, and dielectric and electrical properties of (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ ceramics depending on the sintering temperature is examined by employing X-ray diffraction, field emission scanning electron microscopy, impedance analyzer, and mechanical force system for energy harvesting.     

The unusual case of plastic deformation and high dislocation densities with the cold sintering of the piezoelectric ceramic K0.5Na0.5NbO3

K_0.5Na_0.5NbO₃ (KNN) can be readily densified using the cold sintering process, but despite observing high relative permittivity, the ferroelectric hysteresis is strongly suppressed along with a major suppression in the all-important piezoelectric properties. In this study, KNN is fabricated using a NaOH+KOH transient flux under a uniaxial pressure of 400 MPa and heating to 300 °C for 2 h to drive densification to 93% theoretical. It is only after a secondary heat treatment that we observe improvements of the ferroelectric hysteresis and piezoelectric properties. From a detailed structural-property-processing study using analytical transmission electron microscopy (TEM), X-ray line broadening and high field dielectric characterization methodologies we conclude that there is an unusual in-situ plastic deformation process that takes place in addition to the densification under the cold sintering process. High densities of dislocations within grains were observed that lead to multiple pinning sites that impact both the intrinsic and extrinsic contributions to the high field dielectric and piezoelectric properties. Annealing significantly reduced the dislocation density in the highly defective crystallites, observed directly from the TEM and from the sharpening of the X-ray diffraction peaks, resulting in piezoelectric and ferroelectric properties that approached those of conventionally sintered KNN.     

Green and red upconversion luminescence of Er-doped K0.5Na0.5NbO3 ceramics

Er³⁺ doped K_0.5Na_0.5NbO₃ (KNN) lead-free piezoelectric ceramics were synthesized by the solid-state reaction method. The upconversion emission properties of Er³⁺ doped KNN ceramics were investigated as a function of Er³⁺ concentration and incident pumping power intensity. Bright green (~555 nm) and red (670 nm) upconversion emission bands were obtained under 980 nm excitation at room temperature, which are attributed to (²H_11/2, ⁴S_3/2)→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively. The upconversion emission intensity can be adjusted by changing Er³⁺ concentration, and the mechanism of upconversion processes involve not only a two-photon absorption but also a three-photon absorption. In addition to the admirable intrinsic piezoelectric properties of KNN, this kind of material may have potential application as a multifunctional device by integrating its upconversion and piezoelectric property.     

Cytotoxicity and degradation behavior of potassium sodium niobate piezoelectric ceramics

In the present study, two lead-free piezoelectric ceramics, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) and lithium-doped potassium sodium niobate (Li_0.06K_0.47Na_0.47NbO₃, LKNN), were prepared by a solid-state reaction process. The cytotoxicity evaluation indicated that the cytotoxicity of KNN is low. However, a strength decrease was noted after soaking in saline solution for 7 days. The addition of 6 mol% Li into the KNN improves its density; the strength and piezoelectric coefficient are enhanced consequently. Nevertheless, the cytotoxicity of LKNN is slightly higher than that of KNN. The higher cytotoxicity is related to the release of Li ions. The release of Li ion also induces the degradation of piezoelectric performance.     

Effects of GeO2 Addition on Sintering and Properties of (K0.5Na0.5)NbO3 Ceramics

Lead‐free (K_0.5Na_0.5)NbO₃ (KNN) piezoelectric ceramics doped with different amounts of GeO₂ were prepared and characterized. GeO₂ was found to effectively improve the sinterability and piezoelectric properties of the material. The improvement in the sinterability is ascribed to the formation of a liquid phase, which decreased the sintering temperature from 1080°C to 1010°C. The improvement in the properties is attributed to the replacement of Nb⁵⁺ with Ge⁴⁺ to form acceptor dopants. The following optimized properties were obtained from the KNN ceramic with 0.75 wt% GeO₂: piezoelectric constant (d₃₃) = 126 pC/N, planar electromechanical coupling coefficient (k_p) = 42.8%, mechanical quality factor (Q_m) = 140, and dielectric loss (tanδ) = 3.8%.     

Ferroelectric‒to‒relaxor crossover in KNN‒based lead‒free piezoceramics

This study investigated the phase transition behavior and electrical properties of (K_0.5Na_0.5)(Nb_1-xZr_x)O₃ (KNN?100xZ) and (K_0.5Na_0.5)NbO₃–yBaZrO₃ (KNN–100yBZ) lead–free piezoelectric ceramics. The phase transitions in crystal structures were compared in KNN ceramics between single Zr⁴⁺ doping and Ba²⁺Zr⁴⁺ co?doping. Piezoelectric properties such as the piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) are optimized for KNN?6BZ ceramics and were clarified via the polymorphic phase transition from the orthorhombic to pseudocubic phase. The fitted degree of diffuseness (γ) for a phase transition from the modified Curie–Weiss law indicated that KNN ceramics as ferroelectrics are gradually transformed through BaZrO₃ modification. Accordingly, the enhanced strain properties at y = 0.08 consist of coexisting ferroelectric domains and polar nanoregions that are supported by ferroelectric–to–relaxor crossover in KNN?100BZ ceramics.     

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.     

Textured (K0.5Na0.5)NbO3 ceramics prepared by screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique is successfully applied to alkaline niobate lead-free piezoelectric ceramics. Highly textured (K_0.5Na_0.5)NbO₃ (KNN) ceramics with 〈0 0 1〉 orientation (f = 93%) were fabricated by MLGG technique with plate-like NaNbO₃ templates. The influence of sintering temperature on grain orientation and microstructure was studied. The textured KNN ceramics showed very high piezoelectric constant d₃₃ = 133 pC/N, and high electromechanical coupling factor k_p = 0.54. These properties were superior to those of conventional randomly oriented ceramics, and reach the level of those of textured KNN ceramic prepared by tape-casting technique. Compared with other grain orientation techniques, screen-printing is a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.