K0.5Na0.5NbO3 Research Articles

Photoelectric Interaction and Photochromic Mechanism of K0.5Na0.5NbO3 Ceramics.

Rare-earth materials are widely used in the optical field due to their rich energy-level structures, and if endowed with photoelectric response characteristics, they are expected to be applied to photoelectric multifunctional devices. In this paper, Ho3+- and Yb3+-codoped K0.5Na0.5NbO3 (KNN) ferroelectric ceramics are synthesized, and the up-conversion and down-conversion luminescence intensity as well as the decay degree after photochromism can be controlled by the content of Yb3+. Strong green luminescence visible to the naked eye is realized, and the maximum luminescence decay is more than 50%. 2 s of color change fully indicates that the ceramic has the characteristics of fast response photochromic, under the most efficient wavelength of 405 nm by changing the irradiation wavelength. By incorporating data from thermal and electrical stimulation experiments, a comprehensive photochromic mechanism diagram is constructed, where the photochromic properties are predominantly governed by defects. It is believed that this work will provide a theoretical basis for the design of high-performance photochromic ceramics.

Enhanced comprehensive energy storage properties of lead-free KNN based ceramics through composition optimization strategy

As one of the most potential lead-free dielectric capacitors in pulsed power systems, K0.5Na0.5NbO3 (KNN)-based ceramic possesses comparatively high dielectric breakdown strength (BDS) due to its particular submicron grains. Nonetheless, it is hard to improve both the energy efficiency η and the recoverable energy density Wrec of potassium sodium niobate ceramic at the same time. Herein, a composition optimization strategy is used, Bi(Li0.5Nb0.5)O3 (BLN) as a second component and NaNbO3 (NN) as a third component are introduced into KNN-based ceramic. It is corroborated that BLN content facilitates the growth of polar nano-regions (PNRs), and then strengthens the relaxation behavior, lows the remnant polarization Pr and upgrades the maximum polarization Pmax value of the ceramic. A transition from ferroelectricity to relaxation ferroelectricity occurred and an ultrahigh Wrec of 10.03 J/cm3 with a η of 64.15 % and the BDS of 1140 kV/cm are gained in KNN-0.100BLN-NN ceramic, Wrec of 8.25 J/cm3, η of 71.26 % and the BDS of 840 kV/cm are obtained in KNN-0.075BLN-NN ceramic. Besides, the KNN-0.075BLN-NN ceramic exists not only fine energy storage properties, but also high hardness and good temperature and frequency stability, confirming it’s a feasible material to be applied in pulsed power systems

High Temperature-Insensitive Electrostrain Obtained in (K, Na)NbO3-Based Lead-Free Piezoceramics.

Over the last decades, notable progress is achieved in (K, Na)NbO3 (KNN)-based lead-free piezoceramics. However, more studies are conducted to increase its piezoelectric charge coefficient (d33). For actuator applications, piezoceramics need high electric-field induced strain under low electric fields while maintaining exceptional temperature stability across a wide temperature range. In this study, this work developes Li/Sb-codoped KNN (LKNNS) ceramics with high electrostrain by defect engineering and domain engineering. A remarkable strain of 0.43%, along with a giant d33* value of 2177 pm V-1, is attained in the LKNNS ceramic at 20 kV cm-1. The ceramic exhibits a minimal performance decrease of less than 15% over a temperature range from room temperature to 150 °C. The exceptional strain is attributed to the presence of A-site vacancy-oxygen vacancy ( ) defect dipoles and the increase in nano-domains. The hierarchical domain configuration and defect dipoles impede the switched domains from reverting to their original state as temperature increases, furthermore, the elongated dipole moments of caused by rising temperatures compensate for strain reduction results in exceptional temperature stability. This study provides a model for designing piezoelectric materials with exceptional overall performance under low electric fields and across a wide temperature range.

Enhanced energy storage performance of Bi(Mg0.5Hf0.5)O3 modified K0.5Na0.5NbO3 relaxor ferroelectric ceramics via synergistic strategy

K0.5Na0.5NbO3 (KNN)-based ceramics with large polarization (Pmax) and submicron grain size are considered as promising candidates for dielectric capacitors. However, the existence of significant remnant polarization (Pr) imposes constraints on achieving high recoverable energy storage density (Wrec) and efficiency (η). In this study, we developed and produced ceramic materials by utilizing the composition of (1-x)K0.5Na0.5NbO3-xBi(Mg0.5Hf0.5)O3 ((1-x)KNN-xBMH, x = 0–0.22), employing a viscous polymer rolling technique. We utilized a combined strategy to improve the energy storage capabilities of these materials by manipulating their crystal structures, inducing relaxor behavior, and minimizing microdefects. As a result, we obtained an optimum Wrec of 3.32 J/cm3 and η of 85.37 % at an electric field strength of 350 kV/cm in the x = 0.18 ceramic. Furthermore, excellent temperature stability was observed within the temperature range 20–120 °C, along with frequency stability across frequencies between 5 and 500 Hz. Moreover, at 200 kV/cm in the same composition (i.e., 0.82KNN-0.18BMH), we obtained a power density value as high as 103.13 MW/cm3, accompanied by a fast discharge speed reaching 30.6 ns. Overall, our results demonstrate that the comprehensive energy storage performances exhibited by the 0.82KNN-0.18BMH ceramic make it highly promising for applications in pulsed-power supply.

Growth orientation control in rapid solid-state crystal growth of (K,Na)NbO3 single crystals using plate-like NaNbO3 single crystal particles

The rolling-extended orientation technique and plate-like NaNbO3 (NN) single crystal particles prepared by a single-step molten salt synthesis, both of which have been developed to fabricate (K, Na)NbO3 (KNN) textured ceramics, were utilized to control the growth orientation of KNN single crystals synthesized by a rapid solid-stated crystal growth (RSSCG) method. As the seed crystals, two kinds of NN single crystal particles were synthesized using pure NaCl and KCl-NaCl mixed molten salts. Plate-like KNN single crystals of about 1 cm squares with the upper and lower faces almost parallel to the (100) (001) planes were obtained with a probability exceeding 50% when NN single crystal particles were synthesized from mixed salts and were subsequently thermal-treated again in Na2CO3-NaCl mixed molten salts under appropriate conditions to remove the Bi element, which is known as the suppression factor of the crystal growth. The average crystal growth rate was 0.6–1.2 mm h−1. Controlling the growth orientation of KNN single crystals produced by the SSCG method using seed crystals other than KTaO3 single crystals was successfully accomplished for the first time.

Sol-gel fabrication of transparent ferroelectric (K,Na)NbO3/La0.06Ba0.94SnO3 heterostructure

Lead-free K0.5Na0.5NbO3 (KNN) ferroelectric film and transparent La0.06Ba0.94SnO3 (LBSO) bottom electrode are fabricated on (001)-oriented SrTiO3 (STO) substrate by sol-gel. The characterization results confirm an epitaxial relationship between the films and the substrate, as well as a uniform structure and good crystallization quality of the films. The optical measurement shows that the film heterostructure exhibit a high transmittance with a maximum transmittance of ∼80%. The polarization-electric field (P-E) curves demonstrate that the twice remanent polarization value of the ∼500 nm thick KNN film reaches up to 28 μC/cm2 under an electric field of 800 kV/cm, and the effective piezoelectric strain constant (d33*) is measured as 24.8 pm/V. The dielectric properties of the film are displayed, and the leakage behavior can be divided into three stages of Ohmic conduction, Schottky emission and Poole-Frenkel emission with increasing the applied electric field. This study indicates that transparent lead-free ferroelectric KNN heterostructures can be prepared using a cost-effective sol-gel method and shows promise for future applications.

High energy storage performance of KNN-based relaxor ferroelectrics in multiphase-coexisted superparaelectric state

Although K0.5Na0.5NbO3 (KNN) possesses large maximum polarization and relatively high breakdown strength, the large remnant polarization constrains their practical applications as energy storage materials. In this work, through multi-element doping, (K0.5−0.5xNa0.5−0.5xBix)(Nb1−xSn0.2xZn0.6xTa0.2x)O3 relaxor ferroelectrics were prepared. As the superparaelectric states (SPE) were adjusted to room temperature, orthorhombic, tetragonal, and cubic phases coexisted, accompanied by the highly dynamic polar nanoregions (PNRs). In particular, a high recoverable energy storage density of 4.5 J/cm3 and an energy storage efficiency of 83% were achieved for the x = 0.125 ceramic, with the variations less than 11% and 4%, respectively, in the wide temperature range of 20–180 °C. These results demonstrate that the multiphase PNRs in the SPE state is an effective strategy for improving the energy storage performance of KNN-based ceramics.

Polarized Raman, infrared and dielectric spectra of lead-free K0.5Na0.5NbO3 piezoelectric system: insights from ab-initio theoretical and experimental studies

The K0.5Na0.5NbO3 (KNN) system has emerged as one of the most promising lead-free piezoelectric over the years. In this work, we perform a comprehensive investigation of electronic structure, lattice dynamics and dielectric properties of room temperature phase of KNN by combining ab-initio DFT based theoretical analysis and experimental characterization. We assign the symmetry labels to KNN vibrational modes and obtain ab-initio polarized Raman spectra, Infrared reflectivity, Born-effective charge tensors, oscillator strengths etc. The KNN ceramic samples are prepared using conventional solid-state method and Raman and UV–Vis diffuse reflectance spectra are obtained. The computed Raman spectrum is found to agree well with the experimental spectrum. In particular, the results suggest that the mode in range ∼840–870 cm−1 reported in the experimental studies is longitudinal optical with A1 symmetry. The Raman mode intensities are calculated for different light polarization set-ups that suggests the observation of different symmetry modes in different polarization set-ups. The electronic structure of KNN is investigated and optical absorption spectrum is obtained. Further, the performances of DFT semi-local, meta-GGA and hybrid exchange-correlations functionals, in the estimation of KNN band gaps are investigated. The KNN bandgap computed using GGA-1/2 and HSE06 hybrid functional schemes are found to be in excellent agreement with the experimental value. The COHP, electron localization function and Bader charge analysis is also performed to deduce the nature of chemical bonding in the KNN. Overall, our study provides several bench-mark important results on KNN that have not been reported so far.

Efficiently high energy storage density in Ba2+ ion modified K0·5Na0·5NbO3 (KNN) ferroelectric ceramics for super capacitors

In the present work, Perovskites BaxK0.5-xNa0.5NbO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20) were synthesized via solid-state synthesis technique. XRD analysis confirmed the successful phase formation of the synthesized perovskites. Further, Rietveld refinement revealed the presence of coexisting orthorhombic and tetragonal phases. High-resolution transmission electron microscopy (HRTEM) was employed to explore the lattice fringes and SAED patterns at nano scale level of prepared samples. The morphology was investigated using a field emission scanning electron microscope (FESEM). Dielectric measurements indicated that both the dielectric constant (ε′) and loss tangent (tan δ) exhibited significant dispersion at low frequencies and high temperatures. Notably, the Ba-doped perovskite samples demonstrated enhanced dielectric properties compared to the undoped KNN sample, with Ba0·1K0·4Na0·5NbO3 (x = 0.10) showing optimal dielectric performance, characterized by a dielectric constant (ɛ՛⁓4000) and a tanẟ ⁓7.23 at 100 Hz frequency and room temperature.AC conductivity is observed to increase with increasing both frequency and temperature. DC conductivity exhibited positive temperature coefficient indicating the semiconductor-like behavior of the prepared samples. The ferroelectric behavior was confirmed through P-E hysteresis curves and Ba0·1K0·4Na0·5NbO3 sample revealed the highest energy storage density among all prepared samples making it a suitable candidate for energy storage applications.

Effect of A and B-site ion doping on the structure and properties of KNN-based ceramic coatings

K0.5Na0.5NbO3 (KNN) ceramics have been widely studied for high Curie temperature (Tc) and good electrical properties. The structural and electrical properties of KNN ceramics were significantly improved by A-site and B-site doping modifications. In this paper, KNN-based ceramic coatings doped with Mn, and Sb were prepared by supersonic plasma spraying. The effects of doping ions at A-site and B-site on the microstructure, phase structure, mechanical properties, and electrical properties of KNN-based ceramic coatings were mainly analyzed. The results show that the surface of the three coatings KNN, K0.5Na0.5Nb0.94Sb0.06O3 (KNNS), and K0.49Na0.49Mn0.02NbO3 (KNMN) is relatively flat, uniform distribution of the elements. The surface roughness (Sa) is 3.85 μm, 3.88 μm, and 3.66 μm, respectively. Pores and cracks are present within the coatings, but the KNNS ceramic coatings have significantly fewer pores and cracks compared to the KNN and KNMN ceramic coatings. XRD results show that KNN-based ceramic coatings exhibit single perovskite structure, and both Mn and Sb are successfully introduced into the KNN ceramics without the generation of a second phase. XPS results showed that the valence states of the elements in the KNN ceramic coatings did not change. In KNMN and KNNS ceramic coatings, the valence state of the doped elements did not change. The addition of Mn2+ and Sb3+ reduced the oxygen vacancy in KNN ceramic coatings. Among them, the KNMN ceramic coatings microhardness increased by 45.3 HV0.2, remnant polarization (Pr) increased by 8.1 μC/cm2, the dielectric constant (εr) increased by 107, and dielectric loss greatly reduced, reaching 0.005. The KNMN ceramic coatings microhardness increased by 90.8 HV0.2, dielectric constant increased by 195, Pr increased by 16.7 μC/cm2, dielectric loss is 0.007. These findings demonstrate that doping with Mn2+ and Sb3+ has a positive impact on the performance of KNN ceramic coatings, with Sb3+ having a greater effect.

Studies on the compositional dependent structural and electrical properties of CaTiO3-modified K0.5Na0.5NbO3 piezoelectric system

Lead-free piezoelectric ceramics of (1 − x)K0.5Na0.5NbO3-xCaTiO3 were fabricated, and their crystal structure, microstructure, and electrical properties were systematically studied. Rietveld refinement of the x-ray diffraction data and Raman spectroscopic analyses revealed a composition-dependent structural phase transition: three phase transitions, namely, from a pure orthorhombic phase for x ≤ 0.02 to a mixed phase of orthorhombic and tetragonal phases (0.03 ≤ x ≤ 0.08) and finally another mixed phase of tetragonal + cubic for x = 0.10 and 0.15 at room temperature (RT). The morphological study reveals a decrease in grain size along with a more uniform distribution of grains as the concentration of CaTiO3 (CT) increases; notably, a homogeneous distribution of grains is observed for x = 0.05. The temperature-dependent dielectric properties show two phase transitions, from orthorhombic to tetragonal (TO-T) and tetragonal to cubic (TC), for unmodified K0.5Na0.5NbO3 (KNN). However, both the phase transition temperatures (TO-T and TC) decrease, and the transition peaks broaden with an increase in CT substitution, and for x > 0.06, the TO-T shifted below RT. The broadening of the transition peak at TO-T may be due to the relaxation behavior. Among the prepared samples, the 5 mol. % CT-modified KNN shows the optimum electrical properties (d33 = 114 pC/N, ɛr = 412, and 2Pr = 15.25 μC/cm2) at RT. The enhanced electrical properties for x = 0.05 are due to the coexistence of orthorhombic and tetragonal phases, facilitating easy polarization rotation and flattening of the free energy profile. A phase diagram has been constructed based on the information gathered from the temperature-dependent dielectric measurements, RT x-ray diffraction, and Raman spectroscopy data and is discussed in detail.

Effects of doping with selected rare-earth elements (Eu3+, Dy3+, Eu3+/Dy3+) on the structural characteristics as well as electrical and luminescent properties of K0.5Na0.5NbO3

The effects of adding Eu3+, Dy3+, and Eu3+/Dy3+ rare-earth elements on the microstructural, optical and electrical properties of lead-free piezoelectric materials based on K0.5Na0.5NbO3 (KNN) were investigated. A series of fine powders were synthesized via a modified Pechini route utilizing citric acid. Relatively large amounts of rare-earth elements were successfully incorporated into the crystal lattice of KNN. A combination of X-ray diffraction, X-ray absorption spectroscopy, energy-dispersive X-ray and scanning electron microscopy were used to characterize the structure and chemical composition of the powders and sinters. XRD patterns indicated the formation of a single-phase orthorhombic structure in the co-doped sinter. X-ray absorption spectroscopy revealed the nominal 3+ valence states of Eu and Dy in KNN ceramic sinters. The Mott model and group symmetry theory were used to discuss the characteristics of Raman spectra obtained for the EuDy:KNN ceramics. The ceramics exhibited good dielectric permittivity (ε≃1300) and low dielectric loss (tan δ=0.03) in the temperature range of 170–240°C. The electrical properties of the ceramics showed that the basic mechanism of conduction and relaxation was thermal activation, and that small polaron hopping was responsible for charge carrying. The Eu3+ and Dy3+ ions displayed characteristic red and orange emissions in sinters when excited with light in the wide ultraviolet spectra range (380–420 nm).

High energy storage performance of (1-x)Ba0.5Sr0.5TiO3-xK0.5Na0.5NbO3 ceramics via a combined strategy of fine grains and multiphase polar nanoregions

Electrostatic dielectric capacitors have received great attention due to their ultrafast charging-discharging speed and ultrahigh power density. However, the low energy storage density and efficiency are the main obstacles to their practical applications. In this work, solid solution relaxor ceramics with different proportions of lead-free ferroelectrics Ba0.5Sr0.5TiO3 (BST) and K0.5Na0.5NbO3 (KNN) were prepared. Superior energy storage performance was achieved in the 0.7BST-0.3KNN ceramics with a breakdown strength (Eb) of 510 kV/cm, a recoverable energy storage density (Wrec) of 4.10 J/cm3, and an energy storage efficiency (η) of 80 %, which was fairly stable over the temperature range of 30–100 °C. Since multiple cations with different valence and radius coexist on the equivalent sites (A or B sites) of the solid solutions, the chemical disorder and the corresponding lattice distortion induced the coexistence of cubic, tetragonal, and orthorhombic phases, as well as the appearance of the multiphase polar nanoregions (PNRs), making the ferroelectric hysteresis loops linear. At the same time, ultrafine grains in the ceramics greatly improved the Eb. For achieving excellent energy storage performance, this work demonstrates a potential candidate of (1-x)BST-xKNN lead-free solid solution ceramics and a combined strategy of fine grains and multiphase PNRs.

Facile preparation of KNN thin film with high purity phase and excellent electrical properties

Obtaining high purity alkali niobate (K x Na1-x NbO3) thin films without secondary phase on metal coated traditional silicon (Si) substrates via sol–gel technique has remained great challenges until now. Herein, we report K0.5Na0.5NbO3 (KNN) thin films successfully deposited on Pt/Ti/SiO2/Si(100) substrates by a simply effective sol–gel process. A comprehensive and systematic investigation of processing conditions on the microstructures and electrical properties of spin-coated KNN films was presented. We have found that phase purity and microstructures of KNN films are strongly influenced by content of alkali excess and the annealing temperature. Thin films with an equal excess amount of 10% mol K and Na (KNN1) sintered at 650 °C show high crystallinity with a preferred (100)-orientation degree of 78%, and homogeneous and dense surface with columnar structure and large grain size up to 254 nm. The result of quantitative XPS analysis has proved that the composition of the film is close to the chemical stoichiometry. As a consequence, the obtained KNN1 films exhibit a large dielectric constant of 775 and low dielectric loss of ∼2% in the wide frequency range from 1kHz up to 10MHz as well as the best shape of P−E loops. Furthermore, leakage current density of the film is about 9.45 × 10−5 A cm−2 at E ≈100 kV cm−1.

Lead-free (K0.5Na0.5)NbO3 co-substituted with Ta5+ and Er3+: Structural, optical, dielectric and electrical properties

The lead-free K0.5Na0.5NbO3 (KNN) ceramics co-substituted with 0.05 mol Ta5+ and x mol Er3+ ions (K, Na) (1-3x) ErxNb0.95Ta0.05O3, with x = 0.00 to 0.05, have been successfully manufactured by using the solid-state reaction. The XRD results showed that the ceramics crystalize in pure perovskite with a trace of secondary phase, for low Er levels (x ≤ 0.02). Above this rate, the presence of secondary phases became more significant. The optical band gap energy of the synthesis samples has been determined using UV–vis absorbance spectra using Tauc's relation. The Ta and Er co-substitution leads to decrease the Eg of KNN material. The grain size is found to be minimal ≈490 nm and 240 nm of sintered pellets at x = 0.02 and 0.05 of Er content respectively with a high density. The dielectric study of (K, Na) (1-3x) ErxNb0.95Ta0.05O3 ceramics was carried out and double peaks were observed in relative permittivity versus temperature data for pure KNN and KNNTa corresponding to orthorhombic-tetragonal-cubic phase transitions. While for Er doped-KNNTa samples only a broad ferroelectric-paraelectric phase transition is observed and it shifted to the high temperature with increasing Er content. This broad peak observed in dielectric data for x = 0.02 of Er content is due to inhomogeneity and will certainly open new applications in microwave dielectrics. The ac conductivity variation of KNErNTa ceramics with diverse Er concentration was analyzed according to the charge compensation mechanism. However, for KNNTa, a decrease in conductivity values is attributed to the reduction in leakage current density. Meanwhile, at x = 0.01 and 0.02 of Er concentration, Er-doped KNNTa became semi-conductor. Further investigation revealed that the resistance of grains and grains boundaries are both minimal for x = 0.02 sample which are responsible for the increase of ac conductivity. For high Er concentration ≥0.03, the RG and RGB values increase along with decrease of conductivity value related to the increase in grain size found in SEM micrographs.

Ferroelectric photovoltaic effect engineered by phase transition and defect neutralization in K0.5Na0.5NbO3 film

As an excellent light transmission material, K0.5Na0.5NbO3 (KNN) shows considerable potential in ferroelectric photovoltaics. However, poor leakage and non-intrinsic polarization limit their photoelectric conversion since they play important roles in the generation, separation and transport of photogenerated carriers. This work realizes the simultaneous improvement of bandgap, ferroelectric and leakage by the introduction of charged group BiFeO3 (BFO), which neutralizes the defects and reduces the leakage current of the films by about two orders of magnitude and increases the remanent polarization to 15.4 μC/cm2 due to the phase transition. Meanwhile, the addition of Fe also introduces impurity energy levels into the KNN films and reduces the bandgap. The open-circuit voltage of the 0.98KNN-0.02BFO film is increased by about 2.5 times. This work provides a feasible way to enhance the ferroelectric photovoltaic performance of KNN films and accelerate their application.

Single Crystal (K,Na)Nb)3/CuO Heterostructures for Synergistic Photo-Piezocatalytic Activity

In this study, we designed a heterostructure piezo-photocatalyst composed of Ba, Li doped- (K,Na)NbO3 (KNN) single-crystal microcuboids and CuO nanodots. KNN was selected because of its high piezoelectric coefficient (d33 ), good chemical stability, and environmental friendliness. As a cocatalyst, CuO was considered owing to its advantages such as narrow bandgap, low cost, and nontoxicity. The n-type semiconducting Pb-free KNN single-crystal ferroelectric microcuboids (~30 μm) were synthesized by molten-salt reaction. Then, p-type semiconducting CuO nanodots (less than 5 nm) were uniformly decorated onto KNN microcuboids via deposition of Cu(OH)2, followed by conversion to CuO through calcination. From our heterostructure, it is expected that excellent piezoelectric properties could be obtained from KNN single-crystal microcuboids and a high surface area for catalytic reactions is guaranteed by CuO nanodots that are evenly deposited on the surface of KNN without aggregation. The novel KNN/CuO micro/nano heterostructures exhibited increased current density, and enhanced removal organic dye (RhB) efficiencies under the ultrasonic vibration when compared with bare KNN single-crystal catalysts. Specifically, the reaction rate constant (k) for the photo-piezocatalytic RhB dye degradation of the KNN/CuO micro/nano heterostructure exhibited ~ 0.093 min-1, which is higher when compared with other KNN-based piezo, photo, and photo-piezocatalysts. Additionally, the coupled photo-piezocatalyst for KNN/CuO realized an enhanced RhB dye degradation efficiency of 1.6 times and 48.6 times higher when compared with individual piezocatalytic and photocatalytic dye degradation capabilities, respectively. Our approach could provide insights into the design of heterostructure photo-piezocatalysts containing single-crystal ferroelectrics, which requires the enhancement of the photo-piezocatalytic activities.

An interpretable machine learning strategy for pursuing high piezoelectric coefficients in (K0.5Na0.5)NbO3-based ceramics

Perovskite-type lead-free piezoelectric ceramics allow access to illustrious piezoelectric coefficients (d33) through intricate composition design and experimental modulation. Developing a swift and accurate technology for identifying (K, Na)NbO3 (KNN)-based ceramic compositions with high d33 in exceedingly large “compositional” space will establish an innovative research paradigm surpassing the traditional empirical trial-and-error method. Herein, we demonstrate an interpretable machine learning (ML) framework for quick evaluation of KNN-based ceramics with high d33 based on data from published literature. Specifically, a thorough feature construction was carried out from the global and local dimensions to establish tree regression models with d33 as the target property. Subsequently, the feature-property mapping rules of KNN-based piezoelectric ceramics are further optimized through feature screening. To intuitively understand the correlation mechanisms between ML regression targets and features, the sure independence screening and sparsifying operator (SISSO) method was employed to extract the essential descriptors to explain d33. A straightforward descriptor, e(NMB−MVB)⋅ST/(IDA)2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{e}}^{({{NM}}_{\ ext{B}}-{{MV}}_{\ ext{B}})}\\cdot {ST}/{(I{D}_{\ ext{A}})}^{2}$$\\end{document}, consisting of only four easily accessible parameters, can accelerate the evaluation of a series of novel KNN-based ceramics with high d33 while exhibiting strong theoretical interpretability. This work not only provides a tool for the rapid discovery of high piezoelectric performance in KNN-based ceramics but also offers a data-driven route for the design of property descriptors in perovskites.

Multi‐Length Engineering of (K, Na)NbO3 Films for Lead‐Free Piezoelectric Acoustic Sensors with High Sensitivity

AbstractWith increasing concerns about noise pollution, the pursuit of highly dependable piezoelectric acoustic sensors for real‐time noise monitoring has come to the forefront of scientific research. Lead‐based perovskite piezoelectric films, exemplified by lead zirconate titanate Pb(Zr,Ti)O3 (PZT), surpass traditional piezoelectric materials such as ZnO and AlN in their piezoelectric properties, promising substantial advancements in next‐generation acoustic sensor technologies. However, the toxic nature of lead in PZT materials poses formidable environmental and human health risks. In an unprecedented breakthrough, it presents the pioneering development of an environmentally benign lead‐free piezoelectric Micro‐Electro‐Mechanical System (MEMS) acoustic sensor based on potassium sodium niobate (K,Na)NbO3 (KNN) film. High‐quality <001> textured 3 µm‐thick KNN film is successfully integrated into commercially used Si substrate, rendering exceptional piezoelectricity (transverse piezoelectric coefficients e31* of ≈8.5 C m−2) with satisfactory thermal stability. The atomic‐scale Z‐contrast imaging and piezoresponse force microscopy characterizations reveal that the outstanding piezoresponse originates from the local coexistence of multiple phases and the enhancement of extrinsic piezoelectric contributions from in‐plane polarization anisotropy. Finite element simulation is employed to design the triangular cantilever structure and annular diaphragm structure, each corresponding to different operating bandwidths. The resultant MEMS acoustic sensors stand out with outstanding acoustic performance (the high sensitivity and expansive receiving field of view), which are attributed to the microstructural engineering at multi‐length scales for the excellent piezoelectric properties of KNN film. These features enable sensitive acoustic monitoring in various environments, including large‐scale power grids and urban traffic.

Spontaneous pattern of orthogonal ferroelectric domains in epitaxial KNN films

Lead-free piezoelectric (K, Na)NbO3 (KNN) is considered one of the promising candidates for the replacement of Pb(ZrxTi1−x)O3. Several studies underlined the issue of K and Na volatility with increasing deposition temperatures, leading to high leakage currents in thin films, which still represents a major drawback for applications. This paper shows how epitaxial growth with concomitant preferred orientation of KNN films on niobium-doped strontium titanate (Nb:STO) depends on growth temperature and substrate strain. A preferred out-of-plane polar (001) orientation of KNN is obtained at high temperatures (>600 °C), while (100) orientation is dominant for lower ones. The (001) orientation is forced out-of-plane due to the sizeable in-plane stress derived from a negative lattice mismatch of pseudo-cubic KNN with respect to the underlying cubic (001) Nb:STO substrate. Moreover, we show that K-Na deficiency and high leakage of epitaxial KNN films deposited at high temperatures are accompanied by the appearance of a pattern of orthogonal spontaneous ferroelectric domains aligned to the [100] and [010] directions of Nb:STO. This pattern, visible in secondary electron microscopy, piezoforce response microscopy, and conductive atomic force microscopy images, is uncorrelated to the surface morphology. Supported by reciprocal space mapping by x-ray diffraction, this phenomenon is interpreted as the result of strain relaxation via ferroelectric domain formation related to K-Na deficient films displaying a sizable and increasing compressive strain when grown on Nb:SrTiO3. Our findings suggest that strain engineering strategies in thin films could be used to stabilize specific configurations of piezo- and ferroelectric domains.