Pure KNN Research Articles

Water-mediated production of K0.5Na0.5NbO3 piezoelectric ceramics with cold-sintering assisted route: Enhanced dielectric properties

Cold sintering is an attractive method to densify ceramics, providing the opportunity to produce ferroelectrics efficiently. This is particularly important for cases where ceramics like perovskites are prone to high A-site element volatilization at high temperatures. The resulting uncontrolled creation of defects is often detrimental to the ferroelectric properties. In this work, cold-sintering assisted production of KNN ceramics with water (no corrosive transient liquid) leads to a homogenous, high-density pure KNN with high dielectric permittivity and lower dielectric losses as compared to high-temperature sintering techniques. Our work could, therefore, open a new venue for further modification of defect chemistry in lead-free piezoelectrics.

Experimental determination of electrophoretic deposition parameters and electrical characterization for K 0.5 Na 0.5 NbO3 perovskite thick films for energy harvesting applications

Globally, depleting non-renewable energy resources and environmental pollution are significant challenges. Much research is ongoing on perovskite coatings as a potential replacement. In this work, potassium sodium niobate (KNN) ceramics are fabricated by solid-state method, calcined at ∼850 °C, and electrophoreticcally deposited (EPD) on Ni-substrate uniquely then, sintered at high temperatures ∼1070–1120 °C. The X-ray diffraction and FTIR confirmed the development of a pure KNN perovskite structure and metallic bond groups (-O-Nb-O) present respectively. An increase in the sintering temperature resulted in the pronounced peaks observed in KNN ceramics, confirmed by Raman spectroscopy, and easily observed in SEM having “square” and “circular” morphology with grain growth. The coating thickness was measured around 18–116 μm and increasing deposition rate (0.084–0.337 μm/s) was calculated. The coating roughness (∼813 nm) was confirmed by atomic force microscopy. Complex impedance spectroscopic (CIS) analysis confirmed the high dielectric constant (∼4789) with a high transformation and curie temperature (TO→T ∼ 280 °C & TC ∼ 480 °C), respectively. The increasing conductivity (≥830 μS/m) at higher frequency and temperatures agree with the hopping conduction mechanism which confirmed the negative temperature coefficient of resistance (NTCR). The work holds great significance in sensors, actuators, spintronics, and energy harvesting applications.

Structure, dielectric and ferroelectric Properties of (1-x)K0.5Na0.5NbO3 – xBi0.5Na0.5TiO3(0 ≤ x≤ 0.1) Solid Solution

In this study, we prepared lead-free (1-x)K0.5Na0.5NbO3 – xBi0.5Na0.5TiO3 ceramics (KNNBTx) with x = 0, 0.025, 0.05, 0.075, 0.1 using the conventional solid-state reaction method. Scanning electron microscopy analysis revealed dense ceramics with a notable decrease of the average grain size from 3 μm for x = 0–0.3 μm for x ≥ 0.050. X-ray diffraction pattern refinement unveiled that the pure KNN possessed an orthorhombic Amm2 symmetry. In contrast, the KNNBT0.100 composition displayed a tetragonal P4mm symmetry, while the KNNBT0.025 composition exhibited a monoclinic Pm symmetry, suggesting the stabilization of an intermediate phase between the orthorhombic and tetragonal symmetries for x = 0.025. Dielectric measurements indicated that the introduction of small cations in A and B sites of the KNN matrix induced a diffusive phase transition. According to the modified Curie–Weiss law (named also Uchino and Nomura function), the degree of diffuseness (γ) increased from γ = 1.45 to γ 1.74 with increasing x. Raman spectroscopy revealed a frequency upshift of the vibration modes along with broadening of the modes as the NBT composition increased, indicating increased structural disorder in the perovskite structure. Temperature-dependent Raman spectra exhibited the persistence of certain modes at higher temperatures due to the off-centering of cations at A and B sites, a result of the induced substitution-induced disorder in the KNN matrix. Temperature dependent of Raman modes’ frequency have shown substantial modifications at temperatures close to the critical temperatures identified by dielectric measurements. Ferroelectric measurements demonstrated that the incorporation of NBT into the KNN matrix led to significant alterations in the polarization and ferroelectric characteristics of the parent ceramic (pure KNN). As the NBT content increased, the remanent polarization decreased from 13.1 to 3.6 μC/cm2, and the P-E curves became pinched/distorted, indicating a disruption in the long-range order of KNN due to structural and grain size modifications.

Breakdown performance of potassium‐sodium niobate piezoceramics copped with oxides

AbstractIn the work, the CuO, Fe2O3, ZnO, and Al2O3 are doped in the potassium‐sodium niobate (KNN) to investigate their effects on the breakdown strength (EB) through in situ decomposition reaction on the KNN particles. The results suggest that the EB increases to some extent, thanks to the addition of oxides. Particularly, the addition of ZnO improves the EB by 34.08% compared with pure KNN ceramics. The X‐ray diffractometry, SEM, temperature‐dependent dielectric constant, complex impedance spectrum, breakdown strength test, and energy band structure are employed for the EB investigation. The addition of Fe2O3 and Al2O3 can reduce the grain size of ceramics while CuO leads to grain growth. The relative density, dielectric constant, activation energy (Ea), and relaxation time (τ) of the ceramics are enhanced by the addition of Fe2O3, ZnO and Al2O3. Among these factors, the relative density is helpful for the improvement of the EB. Additionally, the featured energy band structure contributes to such improvement.

Suppression of the ferroelectric phase for (K0.5Na0.5)NbO3 ceramic by A/B-sites disorder for enhancement the energy storage properties and dielectric breakdown strength

In the present study, we propose a new strategy called suppression ferroelectricity of (K0.5Na0.5)NbO3 (KNN) lead-free ceramics for significant enhancement of energy storage properties (ESPs) and dielectric breakdown strength (DBSs). Both of pure KNN and (Na0.6Ag0.1Bi0.1)(Nb0.8Ti0.25)O3 (abbreviate NN-ABT) were synthesized using solid reaction technique. The crystal structure, dielectric and ferroelectric properties of both prepared ceramics were investigated. NN-based ceramics shown ferroelectric to relaxor phase crossover with pseudo-cubic crystal structure at ambient temperature. Present relaxor phase is attributed to substitution of monovalent (K1+) by trivalent (Bi3+) which can cause A-site cation vacancies and charge misfit of KNN lattice. The inhabitation of grain size to sub-micrometer observed by scanning electron microscope and this could be attributed to the lower ionic radius of (Ag1+ = 1.15 Å) and (Bi1+ = 1.38 Å) compared to (K1+ = 1.64 Å) which can suppressed the tolerance factor (τ) from 1.01 for KNN (ideal ferroelectric phase) to τ = 0.85 for NN-ABT . Suppression of the (τ) value can enhanced the degree of the relaxor phase, while inhibition of grain size can improved the DBSs or Eb. Superior enhancement of energy storage properties were achieved in NN-based ceramic due to hybridization between Bi3+ 6 P and O2- 2 P orbitals instead of K1+ 1 S and O2- 2 P orbitals which resulting in increasing the maximum polarization. The results shown that (NN-ABT) is acquired the preeminent of recoverable energy storage density Wrec ∼ 11 J/cm3, energy storage efficiency (ƞ = 89%) at E = 550 kV/cm. Furthermore, the sample shown an excellent thermal and frequency stabilities variation in a wide range of temperature (25–175 °C) and frequency (2–50 Hz). The significant enhancement in energy storage performance of NN-ABT ceramic is attributed to disturb the ferroelectric phase of KNN and forming PNRs of relaxor phase at ambient temperature. Therefore, the present study provides a good guideline for optimizing the ESPs and DBSs of NN-based ceramics to be more applicable for pulsed power and energy storage applications.

Ferroelectric and piezoelectric properties of lead-free Li0.06(K0.5Na0.5)0.94NbO3 thin films

Lead-free ([Formula: see text][Formula: see text])NbO3 (KNN) and [Formula: see text]([Formula: see text][Formula: see text])[Formula: see text]NbO3 (LKNN) thin films were fabricated by a sol-gel method. The effects of Li substitution on crystal structure, microstructure and electrical properties of KNN film were systematically studied. Li doping can enhance the ferroelectric and piezoelectric properties of KNN film. Compared with pure KNN film, the LKNN film possesses larger remanent polarization ([Formula: see text] [Formula: see text] 9.3 [Formula: see text]C/[Formula: see text]) and saturated polarization ([Formula: see text] [Formula: see text] 41.2 [Formula: see text]C/[Formula: see text]) and lower leakage current density ([Formula: see text]A/[Formula: see text] at 200 kV/cm). Meanwhile, a typical butterfly shaped piezoelectric response curve is obtained in the LKNN film with a high piezoelectric coefficient ([Formula: see text] [Formula: see text] 105 pm/V). Excellent fatigue resistance ([Formula: see text][Formula: see text] switching cycles) and aging resistance ([Formula: see text] 180 days) demonstrate the long-term working stability of LKNN film. These findings indicate that KNN-based lead-free piezoelectric films have a broad application prospect in microelectromechanical systems (MEMS).

Enhanced pyroelectric performance in potassium sodium niobate‐based ceramics via multisymmetries coexistence design

AbstractAchieving excellent pyroelectric performance remains a challenge for lead‐free piezoelectric ceramics. To meet the requirements of both an enhanced pyroelectric coefficient at room temperature and good thermal stability during the encapsulation of pyroelectric devices, (1–x)K0.48Na0.52NbO3–xBi0.5Ag0.5ZrO3–0.2%Fe2O3 (KNN–BAZ–Fe) lead‐free ferroelectric ceramics with high Curie temperatures were prepared to obtain improved pyroelectric performance via the coexistence of multiple symmetries. The variation of BAZ content led to the formation of rhombohedral–orthorhombic–tetragonal phase boundary and promoted grain growth, resulting in the best pyroelectric coefficient (p = 5.09 × 10−4 C m−2°C−1) and enhanced figures of merit (Fi = 0.2084 × 10−9 (m V−1), Fv = 0.0142 m2 C−1, Fd = 0.0947 × 10−4 Pa−1/2, and Fe = 17.66 J m−3 K−2) for infrared (IR) detection when x = 0.05. The room‐temperature pyroelectric coefficient obtained in this study is approximately four times that of the pure KNN ceramic and is the maximum value reported for niobate‐based piezoelectric ceramics. Moreover, compared with the poor thermal stability of barium titanate‐ and bismuth sodium titanate‐based ceramics because of their ultralow Curie temperature or thermal depolarization temperature, the ceramics investigated here exhibit much better thermal stability because of their high Curie temperature (TC > 300°C) and diffused phase‐transition behavior, making them more adaptable for practical applications. These results suggest that KNN–xBAZ–Fe ceramics are attractive candidates for applications in the field of IR sensors.

Facile Glycothermal Synthesis of KxNa(1−x)NbO3 Particles

KxNa(1−x)NbO3 particles (KNN, 0 < x < 1) were successfully synthesized through a facile glycothermal method by using KOH, NaOH and Nb2O5 as precursors and 1,4-butanediol as solvent at 200 °C for 12 h. The effects of varying the 1,4-butanediol/deionized water (B/W) volume ratio as solvent on the growth behavior, the morphological evolution, and the particle size of the synthesized KNN particles were investigated. In order to obtain K0.5Na0.5NbO3 with the morphotropic phase boundary (MPB) at the potassium content of x ≈ 0.5, the effect of varying K+/Na+ molar ratio on the composition of the obtained KNN particles was investigated. The crystal phase structure, morphology, particle size, chemical composition, and thermal behavior of the obtained particle samples were characterized using XRD, FE-SEM, EDS, TG, FT-IR, PSA, and TEM. The pure orthorhombic KNN particle close to NaNbO3 phase was obtained at the same concentration K+/Na+ of 1.0/1.0 and [K++Na+]/Nb molar ratio of 2.0/0.1. The synthesized K0.01Na0.99NbO3 particle exhibited a hexahedron shape with an average crystallite size of approximately 400 nm by glycothermal treated at 200 °C for 12 h. It is also demonstrated that the size of Na-rich KNN particles was decreased from 15 µm to 400 nm with increasing 1,4-butanediol content at various reaction conditions such as the volume ratio of B/W and can be controlled by 1,4-butanediol with an additive of water. Until the molar ratio of K+/Na+ reaches 1.6/0.4, the obtained particles have produced a Na-rich KNN phase, whereas when the molar ratio of K+/Na+ is 1.8/0.2, the particles could obtain a K-rich KNN phase. The results revealed that single-phase K0.5Na0.5NbO3 particles could be obtained at a relatively narrow molar ratio of K+/Na+ to 1.7/0.3. The particles with weakened agglomerate could obtain the average particle size of approximately 400 nm and a hexahedron shape. In comparison with the traditional hydrothermal method, the glycothermal method has been confirmed to be a more efficient method in controlling the particle size of KNN particles from micro- to sub-micron.

Improved ferroelectric, piezoelectric, and dielectric properties in pure KNN translucent ceramics by optimizing the normal sintering method

In this study, it is reported that various properties can be selectively derived in a pure (K0.5Na0.5)NbO3, KNN ceramics through optimizing the sintering temperature by the conventional sintering method. High piezoelectric, ferroelectric, and dielectric properties such as d33 = 127 pC/N, Pr = 31 μC/cm2, and εr = 767 are obtained at the sintering temperature of 1100 °C. On the contrary, the specimen sintered at 1130 °C does not show high piezoelectric and ferroelectric properties, but it is translucent with a transmittance of 22% and 57% at the wavelength of 800 and 1600 nm respectively and shows a very high dielectric constant εr of 881. The origin of the high piezoelectric constant owes to large remanent polarization and dielectric constant, and dense microstructure with uniform distribution of large grains with the conjunction of relatively large crystal anisotropy. On the other hand, dense microstructure with almost no porosity, highly compacted grain boundaries, uniform distribution of grains, and relatively low crystalline anisotropy are responsible for the translucency and large dielectric constant of the ceramic specimens. This study demonstrates that the lead-free KNN ceramic has the potential to show multiple noteworthy properties such as piezoelectric, ferroelectric, dielectric, and transparent properties. This work provides a pure KNN ceramic simultaneously with high piezoelectric and transparent characteristics prepared only by using the conventional sintering method at a moderate sintering temperature for the first time in the literature.

Hydrothermal synthesis of potassium–sodium niobate powders

AbstractPotassium–sodium niobates (KxNa1−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.

Electrical Properties of Li+-Doped Potassium Sodium Niobate Coating Prepared by Supersonic Plasma Spraying

The current work aims to compare the effects of systematic A-site substitutions on the electrical properties of potassium sodium niobate (KNN)-based coating. The A-site elements were replaced by Li+ to form (K0.4675Na0.4675Li0.065) NbO3 (KNLN). The pure KNN coating and the Li+-doped potassium sodium niobate (KNLN) coating with dense morphology and single perovskite structure were successfully prepared by supersonic plasma spraying, and the phase composition, microscopic morphology and electrical properties of the two coatings were compared and analyzed in detail by XRD, XPS, three-dimensional morphology and SEM on an Agilent 4294A (Santa Clara, CA, USA) and FE-5000 wide-range ferroelectric performance tester. The results show that: as the polarization voltage increases, the pure KNN coating is flatter and fuller, but the leakage current is large. The KNLN coating has a relatively long hysteresis loop and is easily polarized. The domain deflection responds faster to the external electric field, and the resistance of the domain wall motion to the external electric field is small. The dielectric constant of KNLN coating is 375, which is much higher than that of the pure KNN coating with 125, and the dielectric loss is stable at 0.01, which is lower than that of pure KNN coating at 0.1–0.35. This is because Li+ doping has successfully constructed a polycrystalline phase boundary in which O-T phases coexist, and has higher dielectric properties, piezoelectric properties and ferroelectric properties. At the same time, due to the high-temperature acceleration process in supersonic plasma spraying, the violent volatilization of the alkaline elements Li+, Na+ and K+ leads to the presence of oxygen vacancies and part of Nb4+ in the coating, which seriously affects the electrical properties of the coating.

Study of dielectric properties of lead-free multiferroic KNN/22.5 BaFe12 O19 composites

A multiferroic (K_0.5 Na_0.5 NbO_3) -22.5% vol.(〖BaFe〗_12 O_19) composite obtained successfully by conventional sintering (abbreviated as CS) method at 1080 ℃. To compare the dielectric properties of the samples, lead-free K_0.5 Na_0.5 NbO_3 (abbreviated as KNN) piezoceramics were prepared by CS method at 1125℃. The structure and morphology have been determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and also the dielectric properties of samples were carried out. In the X-ray spectra of composite samples, all peaks related to the KNN and BFO phases, were observed without any trace of the second phase. In the SEM images of the composite, the distinct cubic morphology of the KNN phase indicating the formation of the perovskite structure of the compound, and the polygonal grains of the BFO phase were observed. The values of relative density, dielectric coefficient, and loss factor of the lead-free KNN ceramic at a sintering temperature of 1125℃ was measured to be 91%, 390, and 0.02, respectively. These values at a sintering temperature of 1080℃ were about 92%, 200 and 0.18 respectively. Although the dielectric properties of KNN-BFO composite are lower than those of pure KNN, the presence of magnetic phase can create the magnetic properties and consequently multiferroicity in the KNN-BFO composite The dielectric properties also confirm that this composite can be considered as a new multiferroic composite.

Enhancing photostriction in KNN-based ceramics by constructing the morphotropic phase boundary and narrowing the energy band gap

Lead-free ferroelectric ceramics exhibiting photostriction effect has attracted a lot of attention in the past decade. Herein, we fabricated a series of (1−x)K0.5Na0.5NbO3-x(La0.51Na0.49)(Zr0.54Ni0.46)O3 ((1−x)KNN-xLNNZ) ceramics by traditional solid-state synthesis method aiming to improve photostriction. The addition of LNNZ in KNN has led to significant changes in phase structure and grain size. The ceramics with a composition of 0.97KNN-0.03LNNZ has shown a narrow bandgap ~2.43 eV, large piezoelectric coefficient (~209 pC/N), low dielectric loss (0.021 at 1 kHz) and high remnant polarization (~24 μC/cm2). Further, 0.97KNN-0.03LNNZ also exhibits a considerable photostrictive coefficient (~1.83 × 10−9 m2W−1), which is attributed to the combined effect of significantly narrow bandgap along with the morphotropic phase boundary. Further, all the substituted samples show distinct red shift of v1 mode compared to the pure KNN, which suggest a new kind of distortion introduced in (Nb, Zr, Ni)O6 octahedron for KNN-based ceramics under variable power laser excitation. The obtained results indicate that LNNZ-substituted KNN ceramics can serve as a potential material for the fabrication of optomechanical devices.

Ultrahigh electro-strain in acceptor-doped KNN lead-free piezoelectric ceramics via defect engineering

Defect dipoles in acceptor-doped perovskite-based ferroelectric ceramics play an important role in piezoelectric properties. In this work, the acceptor, such as Fe2+ and Cu+, -doped (K0.5N0.5)NbO3 (KNN) ceramics are fabricated in reduced atmosphere by the conventional solid-state reaction method. By tailoring the strongly polarized defect dipoles through poling and aging processes, ultrahigh electro-strain and large signal piezoelectric d33* values can be achieved. The electro-strains of 0.41% and 0.50% with d33* ~820 pm/V and ~1000 pm/V are obtained in Fe2+-doped and Cu+-doped KNN samples, respectively, which are 4-fold and 5-fold larger than that of pure KNN. Of particular interest is that the electro-strain and d33* values are found to improve with increasing temperature up to 140°C, accompanied with low strain hysteresis of 10%-20%. A detailed mechanism considering the interaction between defect dipole polarization and spontaneous polarization after poling and aging processes is proposed to explain the observed phenomena, which provides a good paradigm for achieving high piezoelectric response by defect engineering in perovskite-based ferroelectric ceramics.

Synthesis of KNN powders: Scaling effect of the milling step

The scaled-up production of lead-free potassium sodium niobate (KNN)-based piezoceramics still suffers from poor reliability and reproducibility issues that hinder their penetration into the piezoelectric market. To address this issue, pure KNN ceramics were prepared in this work by mechanochemical activation of the powders, followed by calcination at 700 °C, obtaining high density sintered pellets featuring fine and uniform-grain-size. Results show that both cumulative kinetic energy released in the system and hindrance factor, the latter being paramount, must be properly taken into account when the synthesis is scaled-up. Mechanochemical activation strongly effects both stoichiometry and morphology of the final KNN powders during the milling step, which in turn affects the piezoelectric properties of the sintered pellets. This work definitely proves that proper design and control of the powder synthesis process is a valuable tool in the scaled-up production of KNN powders for the production of reliable and performant KNN ceramics.

Effects of LaScO3 doping on structure, dielectric, and piezoelectric properties of K0.5Na0.5NbO3 piezoceramics

Lead-free (1 − x)K0.5Na0.5NbO3–xLaScO3 (abbreviated as KNN–LSO) ceramic with (x = 0–0.10) was prepared via the solid-state synthesis route. The changes in the microstructure, crystal structure, dielectric, piezoelectric, and ferroelectric properties were analyzed for all compositions. The Rietveld refinement of powder x-ray diffraction suggested the variation in the crystal structure from orthorhombic for pure KNN to a mixed phase of tetragonal and orthorhombic for x ≥ 0.03. Temperature-dependent dielectric measurements also confirmed the change in the crystal structure with the increase in the LSO content, and the tetragonal to cubic TT-C phase transition temperature was found to shift toward room temperature. Interestingly, the temperature of the orthorhombic to tetragonal (TO-T) phase transition remained almost invariant in the LSO-doped sample (x ≥ 0.02), and a frequency dispersion in TO-T, typical for a relaxor ferroelectric, was observed. The frequency variation of temperature of the dielectric maximum Tm (∼426 K at 10 kHz and ∼444 K at 1 MHz) for x = 0.05 sample was fitted using the Vogel–Fulcher relation. This was further corroborated by the appearance of a slim hysteresis in the polarization–electric field plot. The impedance studies suggested that non-Debye relaxation and conduction mechanisms were associated with the thermal activation of oxygen vacancy that was rationalized by similar values of activation energy obtained from Arrhenius fitting for all samples. Current–voltage (I–V) measurements illustrate the reduction in room temperature DC conductivity with the increase in LSO doping. The highest room temperature piezoelectric charge coefficient (d33) value of 130 pC/N was observed for the composition corresponding to x = 0.01.

Ultrahigh storage density achieved with (1-x)KNN-xBZN ceramics

A series of (1-x)K0.5Na0.5NbO3-xBa(Zn1/3Nb2/3)O3 ((1-x)KNN-xBZN) nanostructural ceramics was successfully synthesised via solid-state reactions. These nanostructural ceramics exhibited high energy storage density compared with pure KNN ceramics. Further analysis of their dielectric/ferroelectric properties and structures revealed that the addition of BZN alloy disrupted the long-range order of the ferroelectric lattice of pure KNN and favoured the formation of ferroelectric islands and/or polar nano-regions. Consequently, the nanostructured ceramic with x = 0.05 exhibited ultrahigh energy storage density, W, of approximately 9.14 J/cm3 and recoverable energy storage density, Wrec, of approximately 4.87 J/cm3 under a fairly low applied electrical field (220 kV/cm). These values exceed the highest values ever reported for KNN-based bulk ceramics. In addition, both excellent fatigue endurance (105 cycles) and temperature stability (Δε'/ε100°C < 15 % in the range 30–390 °C) were realised with the 0.97KNN-0.03BZN ceramic. Their excellent energy storage properties render KNN-based ceramics potential candidates for application in pulsed-power systems.

Microstructures, piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K0.5Na0.5)NbO3 were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d33 ∼122 pC/N, kp ∼36 % and Qm ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.

Ultrahigh Electro-Strain in Acceptor-Doped KNN Lead-Free Piezoelectric Ceramics Via Defect Engineering

Defect dipoles in acceptor-doped perovskite-based ferroelectric ceramics play an important role in piezoelectric properties. In this work, the acceptor, such as Fe2+ and Cu+, -doped (K0.5N0.5)NbO3 (KNN) ceramics are fabricated in reduced atmosphere by the conventional solid-state reaction method. By tailoring the strongly polarized defect dipoles through poling and aging processes, ultrahigh electro-strain and large signal piezoelectric d33* values can be achieved. The electro-strains of 0.41% and 0.50% with d33* ~820 pm/V and ~1000 pm/V are obtained in Fe2+-doped and Cu+-doped KNN samples, respectively, which are 4-fold and 5-fold larger than that of pure KNN. Of particular interest is that the electro-strain and d33* values are found to improve with increasing temperature up to 140 °C, accompanied with low strain hysteresis of 10%-20%. A detailed mechanism considering the interaction between defect dipole polarization and spontaneous polarization after poling and aging processes is proposed to explain the observed phenomena, which provides a good paradigm for achieving high piezoelectric response by defect engineering in perovskite-based ferroelectric ceramics.

Synthesis and enhanced piezophotocatalytic activity of Ag2O/K0.5Na0.5NbO3 composites

Ag2O/K0.5Na0.5NbO3 (Ag2O/KNN) piezophotocatalysts with different weight ratios of Ag2O to KNN were synthesized by a facile two-step method involving hydrothermal and precipitation methods. Ag2O particles were attached on the surface of KNN via a precipitation method. Ag2O/KNN composites exhibit better photoactivity than pure Ag2O and KNN under visible light irradiation. Moreover, under both irradiations of visible light and ultrasound, the piezophotocatalytic activity was further enhanced in comparison with photoactivity. Based on the experimental results, the improved piezophotocatalytic activities of Ag2O/KNN composites should be attributed to the formation of p-n heterojunction between Ag2O and KNN and the generation of external electric field of KNN, which leads to high separation rate of photo-induced charges. The photostability and corresponding piezophotocatalytic mechanism were discussed.