Composition segregation and grain growth in (K0.5Na0.5)NbO3 piezoceramics prepared by two-step cold sintering process

Cold sintering process (CSP) is a new method to prepare ceramics under quite low temperature. In this work, two-step CSP under different pressures was employed to prepare (K_0.5Na_0.5)NbO₃ (KNN) ceramics. The density of KNN green pellets can be raised by enhancing the pressure of second-step CSP. Energy-dispersive spectroscopy reveals the composition segregation of A-site cations in large grains. The dissolution rate of K⁺ in an aqueous medium is faster than Na⁺, and high pressure can accelerate K⁺ dissolution, resulting in more Na⁺ in some grains. Besides, the diffusion rate of Na⁺ in grains is better than K⁺, which promote the grains growth. Finally, the piezoelectric property is improved even with low ceramic density due to the larger grains, which possess the higher performance composition. This result demonstrates that the pressure and inhomogeneous dissolution of alkali metal ions among CSP play an important role in grain growth and piezoelectric enhancement.     

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.     

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.     

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.     

Enhanced Piezoelectric Properties and Thermal Stability in the (K0.5Na0.5)NbO3:ZnO Lead‐Free Piezoelectric Composites

The piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN) are normally enhanced by chemical substitutions or doping to form solid solutions. In this study, we report that the piezoelectric properties of KNN and thermal stability of piezoelectric coefficient d₃₃ can be both enhanced by forming the composite of KNN:ZnO. The d₃₃ of KNN:0.2ZnO can be improved to 110 pC/N by introducing the ZnO nanoparticles, which is better than the pure KNN (d₃₃ = 85 pC/N). The Curie temperature (T_C = 407°C) remains well comparable to the pure KNN (T_C = 408°C). Furthermore, the thermal stability of both remanent polarization (P_r) and piezoelectric parameter (d₃₃) is improved. The enhanced thermal stability could be related to the induced built‐in electric field or the enhanced sinterability by the addition of ZnO. The present results may help to optimize the piezoelectric properties of lead‐free materials by forming composite.     

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.     

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.     

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.     

Effect of cobalt doping on the sintering mechanisms of the lead-free piezoceramic (Bi0.5Na0.5)TiO3

In the search for lead-free piezoelectric ceramics, such as potassium sodium niobate, (K_0.5Na_0.5)NbO₃ (KNN), and bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT), high sintering temperatures and the associated volatilization of cations represent a major obstacle to achieve well performing materials. In this study, we investigated the effect of cobalt on the sintering behavior of BNT using in situ thermo-optical dilatometry. The addition of cobalt significantly reduced the sintering temperature at which fully dense ceramic bodies are obtained. This is accomplished by a dual effect of the dopant which facilitates oxygen diffusion: a fraction of the available Ti forms a secondary cobaltous phase. Instead of Ti, some Co is incorporated into BNT at the Ti site, causing oxygen vacancies for charge balancing. To a small degree, the dopant induces liquid phase sintering. At high sintering temperatures, swelling was observed, which was attributed to oxygen release caused by the valence transition from Co³⁺ to Co²⁺.     

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.     

Reversible luminescence modulation and temperature-sensing properties of Pr3+/Yb3+ codoped K0.5Na0.5NbO3 ceramics

In this work, we have prepared a novel (K_0.5Na_0.5)_0.99-xPr_xYb_0.01NbO₃ (abbreviated as KNN:xPr³⁺/0.01Yb³⁺, x = 0.0006, 0.0008, 0.001, 0.002, 0.003, and 0.004) ceramics, which possess visible UC emissions, photochromic (PC) and optical thermometric properties. Under the excitation of a 980-nm diode laser, all the samples show the featured emissions of Pr³⁺ ions and the UC emission intensity is greatly dependent on the Pr³⁺ doping content. The optimal UC luminescence intensity is obtained at x = 0.001. All the prepared samples show a strong PC reaction, and a large luminescence quenching degree (ΔR_t) of 74.94% is found. The optical thermometric properties of both the irradiated and unirradiated KNN:0.001Pr³⁺/0.01Yb³⁺ ceramics in the temperature range of 123-573 K have been investigated via measuring the temperature-dependent UC emission spectra of green emissions, which originate from the two ³P₁ and ³P₀ thermally coupled levels. It has been found that the prepared samples have both excellent PC behaviors and temperature-sensing performances. These results suggest that the KNN:xPr³⁺/0.01Yb³⁺ ceramics are promising candidates for the applications in PC reaction and thermometers.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

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.     

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.     

Energy-storage properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 ceramics fabricated by wet-chemical method

0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ (BNTBT) and KNbO₃ (KN) powders with average particle size of ∼50 nm and ∼300 nm were synthesized by sol-gel method and hydrothermal method, respectively. Then, (1 − x)(BNTBT)-xKN (BNTBT-KN, x = 0, 0.01, 0.03, 0.05, 0.07) ceramic samples were prepared using these two powder precursors. The structure, dielectric and energy-storage properties of BNTBT-KN ceramics were comprehensively investigated. All the ceramic samples were in single perovskite structure, indicating that KN can completely dissolve into BNTBT within the studied composition range. BNTBT-KN ceramics exhibited a high dielectric constant at room temperature, being in the order of 1430–1550. Ferroelectric hysteresis loops at room temperature became more slim with the increase of KN content, which largely improved energy-storage density and efficiency. For the composition of x = 0.05, the maximum recoverable energy-storage density reached 1.72 J/cm³ under 16.8 kV/mm, which is superior to linear dielectrics and even some Pb-based systems. All these results demonstrate that 0.95BNTBT-0.05KN fabricated by wet-chemical method is a promising lead-free dielectric material for energy-storage capacitors.     

CO<Subscript>2</Subscript> retention ability on alkali cation exchanged titanium silicate,ETS-10

ETS-10 was ion exchanged by various alkali cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and the BET surface area and pore volume was exactly consistent with cationic size; that is, in the order of Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺. It was observed that a single point adsorption capacity was inversely proportional to cationic size. The largest CO₂ capacity was observed for Li⁺-ETS-10 and it is attributed to greater cation–quadrupole interactions with CO₂ than larger cation. The results also suggests that as the CO₂ loading is increased, the accessibility of adsorbing CO₂ to framework basic O⁻ sites should have become difficult with the increase in cationic size due to the blocking effect by extra-framework CO₂-M⁺. The slight decrease in the slope of adsorption capacity with temperature, especially beyond 373 K for Li⁺-ETS-10 and K⁺-ETS-10 suggests that the adsorption of CO₂ on small alkali cation exchanged-ETS-10 at high temperature is somewhat associated with basic oxygen anion sites in framework due to the existence of large pore. The CO₂-TPD results show that the amount of desorbed CO₂ at higher temperature was proportionally increased due to the increased basicity of oxygen anions in framework. It also shows that the desorption temperature associated with alkali cations in extra-framework (corresponding to low temperature desorption peak) has been lowered with the increase in cationic size, indicating weak cation–quadrupole interactions with CO₂ for larger cations.     

Hydrothermal synthesis of potassium–sodium niobate powders

Potassium–sodium niobates (K_xNa_1−xNbO₃, 0 < x < 1, KNN) were hydrothermally synthesized under varying alkaline ratios (K⁺/Na⁺), total hydroxide concentration, reaction temperature, and time. Compositional surveys were developed by using Rietveld analyses derived quantitative volume fractions. The data demonstrated that phase pure KNN synthesis can be achieved by reacting the niobium source with the hydroxide solution having 6 M total hydroxide concentration, cation ratio (K⁺/Na⁺) of above 6 at temperatures ≥200°C for 24 h. Dissolution–precipitation events through intermediate products including hexaniobates were postulated as a plausible formation mechanism. It was shown also that the single-phase KNN approaching the morphotropic phase boundary (MPB) could be obtained by further incorporation of sodium ions into the crystal via post-annealing at 800°C/2 h, following the hydrothermal synthesis.     

Impedance Spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 Ceramics Modified with (K0.5Na0.5)NbO3

The electrical and dielectric properties of (1 ? x)(0.94Bi_1/2Na_1/2TiO₃–0.06BaTiO₃)–x(K_0.5Na_0.5NbO₃) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.     

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.     

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.