K1-xNax NbO3 Research Articles

Phase Structures, Electromechanical Responses, and Electrocaloric Effects in K0.5Na0.5NbO3 Epitaxial Film Controlled by Non-Isometric Misfit Strain

Environmentally friendly lead-free K1-xNaxNbO3 (KNN) ceramics possess electromechanical properties comparable to lead-based ferroelectric materials but cannot meet the needs of device miniaturization, and the corresponding thin films lack theoretical and experimental studies. To this end, we developed the nonlinear phenomenological theory for ferroelectric materials to study the effects of non-equiaxed misfit strain on the phase structure, electromechanical properties, and electrical response of K0.5Na0.5NbO3 epitaxial films. We constructed in-plane misfit strain (u1−u2) phase diagrams. The results show that K0.5Na0.5NbO3 epitaxial film under non-equiaxed in-plane strain can exhibit abundant phase structures, including orthorhombic a1c, a2c, and a1a2 phases, tetragonal a1, a2, and c phases, and monoclinic r12 phases. Moreover, in the vicinity of a2c−r12, a1c−c, and a1a2−a2 phase boundaries, K0.5Na0.5NbO3 epitaxial films exhibit excellent dielectric constant ε11, while at a2c−r12 and a1c−c phase boundaries, a significant piezoelectric coefficient d15 is observed. It was also found that high permittivity ε33 and piezoelectric coefficients d33 exist near the a2c−a2, a1a2−r12, and a1c−a1 phase boundaries due to the existence of polymorphic phase boundary (PPB) in the KNN system, which makes it easy to polarize near the phase boundaries, and the polarizability changes suddenly, leading to electromechanical enhancement. In addition, the results show that the K0.5Na0.5NbO3 thin films possess a large electrocaloric response at the phase boundary at the a1a2−r12 and a1c−a1 phase boundaries. The maximum adiabatic temperature change ΔT is about 3.62 K when the electric field change is 30 MV/m at room temperature, which is significantly enhanced compared with equiaxed strain. This study provides theoretical guidance for obtaining K1−xNaxNbO3 epitaxial thin films with excellent properties.

Machine learning aided accelerated prediction and experimental validation of functional properties of K1-xNaxNbO3-based piezoelectric ceramics

The functional properties of piezoelectric ceramics are vital to design materials for energy harvesting applications. In the present study, to accelerate the design process with prediction of functional properties for a composition, a machine learning (ML) model is developed, which predicts (ϵr, TC, d33) values for K1-xNaxNbO3(KNN)-based materials. Simultaneously, important features were identified which directly influence the functional properties such as Martynov and Batsanov (MB) electronegativity, ionic radii, atomic volume of elements, and atomic polarization. The ML model with the best performance is implemented to predict the functional properties of KNN-based ceramics. The measured functional properties namely, d33, TC and ϵr for hydrothermally synthesized KNN10 composition are found to be 131 pC/N, 415 °C and 428.4 respectively, which are consistent with the ML prediction. In addition, the proposed model has extended to predict the properties of KNN-ceramics with series doping of elements at A-site (lithium) and B-site (antimony), and it is in good agreement with the trend of experimental outcomes from literatures. Finally, this work discussed the feature-oriented guidelines for optimizing the synthesis of KNN ceramics.

Elastic moduli of potassium sodium niobate ceramics: Impact of spark plasma texturing

An alternative sintering methodology, Spark Plasma Texturing (SPT), allowed us to prepare 98.0% dense lead-free piezoelectric (K1-xNax)NbO3 (KNN) ceramics with a homogenous microstructure of small grains. In this work, Resonant Ultrasound Spectroscopy (RUS) was used to measure bulk, K, and shear, G, moduli of SPT KNN for comparison with conventionally sintered (CS) KNN. SPT fabrication resulted in the highest elastic moduli K = 120.3 GPa (127.7 GPa for 100% density) and G = 45.47 GPa (47.2 GPa for 100% density) ever reported for KNN ceramics. Moreover, by collecting RUS data in situ as a function of temperature, phase transitions at temperatures of 464 and 688 K for CS KNN, and 443 and 643 K for SPT KNN ceramics were determined. Low acoustic loss in the stability field of the cubic phase indicated that the ceramics were mechanically robust, while high loss in the stability fields of the tetragonal and orthorhombic structures demonstrated high mobility of ferroelastic domain walls.

Growth and piezoelectric properties of amorphous and crystalline (K1−xNax)NbO3−based thin films

(K1−xNax)NbO3 (KNN)-based piezoelectric thin films have been extensively studied for application in micro-electromechanical systems. However, growing homogenous crystalline (K1−xNax)NbO3 (CKNN) films with good piezoelectric properties is difficult because Na2O and K2O evaporate during the growth process at high temperatures. Recently, amorphous (K1−xNax)NbO3 (AKNN) films containing KNN nanocrystals with good piezoelectric properties have been fabricated at low temperatures. Furthermore, [001]-oriented crystalline (K1−xNax)NbO3 (OCKNN) films with excellent piezoelectric properties have been grown at low temperatures (≤ 350 °C) using a metal-oxide nanosheet seed layer. This novel method is excellent for the growth of homogeneous KNN thin films. These films were deposited on a polymer substrate; thus, they can be utilized in future flexible electronic devices. In this study, the structural and piezoelectric properties of AKNN- and CKNN-based films fabricated at high temperatures, along with the growth process and application of OCKNN-based thin films at low temperatures, are reviewed.

0 0 1]-oriented crystalline Potassium-Sodium Niobate thin film fabricated at low temperature for use in piezoelectric energy harvester

In this study, 1.0-μm thick crystalline (K1-xNax)NbO3 (KNN) films were deposited onto various substrates at 350 °C for use in piezoelectric energy harvesters (PEHs). A Sr2Nb3O10 (SN) nanosheet monolayer fabricated on these substrates acted as the seed-layer for the formation of a [001]-oriented crystalline KNN film at 350 °C. The [001]-oriented KNN films deposited on a SN/Ni substrate exhibited smaller εr (278) and larger d33 (166 pm/V) values than those of a randomly oriented crystalline KNN film. Because the output power of the PEH is generally proportional to d332/εr, a [001]-oriented KNN thin film is a good material for use in a PEH. A KNN/SN/Ni PEH shows a large output power density of 20 μW/mm3 (2.9 μW) at 1.0 MΩ based on a soft tapping motion of the finger. The KNN/SN fabricated on the Ni substrate was transferred to a flexible and transparent ITO/polyethylene naphthalate (I-PEN) substrate. The transparent KNN/SN/I-PEN PEH exhibited a promising output power density of 7.02 μW/mm3 with a maximum output voltage of 4.2 V, indicating its applicability as a power source of an electrochromic smart glass window.

Selective Phase Control of Dopant-Free Potassium Sodium Niobate Perovskites in Solution.

As one of the perovskite families, potassium sodium niobates (K1-xNax)NbO3 (KNN) have been gaining tremendous attention due to their various functional properties which can be largely determined by their crystallographic phase and composition. However, a selective evolution of different phases for KNN with controlled composition can be difficult to achieve, especially in solution chemical synthesis because of its strong tendency to stabilize into orthorhombic phase at conventional synthetic temperature. We herein developed a facile solution approach to control the phase and composition of dopant-free KNN particles selectively through the modification of reaction parameters. A conventional hydrothermal synthesis method yielded orthorhombic KNN particles, while the monoclinic phase, which has never been observed in a bulk counterpart, was kinetically generated by the compositional modification of an intermediate phase under a high-intensity ultrasound irradiation. Cubic KNN particles were stabilized when ethylene glycol was used as a co-solvent together with deionized water through bonding between ethylene glycol molecules and the surface of the KNN. Composite-structured piezoelectric harvesters were fabricated using each phase of KNN particles and the β-phase poly(vinylidene fluoride-co-trifluoroethylene) polymer. Maximum output power was found for the harvester containing orthorhombic KNN particles. This facile synthetic methodology could pave a new pathway for fabricating numerous phase-controlled materials.

Effect of gamma irradiation on (K,Na,Li)(Ta,Nb)O3–CaZrO3 lead-free ferroelectric film grown on La0.67Ba0.33MnO3 and La0.67Ca0.33MnO3 conductive oxide electrode

(K1-xNax)NbO3 (KNN)-based lead-free ferroelectric materials featured with the large piezoelectric coefficient, has long been considered as a very promising candidate to replace traditional lead-based ferroelectric materials. Irradiation resistance of materials is important for the application in strong irradiation environments, especially possible application in irradiation resistance devices. In this paper, 0.95(K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3-0.05CaZrO3 (KNN-LT-CZ) lead-free thin films have been grown on SrTiO3(001) substrates with La0.67Ca0.33MnO3 (LCMO) and La0.67Ba0.33MnO3 (LBMO) conductive oxide as bottom electrodes and metallic Pt as top ones. As large as 21% and 26% remnant polarization decay are distinguished in Pt/KNN-LT-CZ/LCMO and Pt/KNN-LT-CZ/LBMO films respectively, before and after gamma ray irradiation of 50 Mrad(Si). The irradiation even induced exotic current behavior in the latter ones, while the former ones exhibit only 10% decrease of the dielectric constant. The KNN-LT-CZ film grown on conductive metal oxides is associated with favorable irradiation resistance which would eventually benefit future design optimization in devices applied in more tough environments.

Ferroelectric domain structures and temperature-misfit strain phase diagrams of K1-xNaxNbO3 thin films: A phase-field study

Potassium-sodium niobate K1-xNaxNbO3 (KNN) is one of the most promising lead-free piezoelectric materials. While there have been many studies on the microstructures and properties of KNN ceramics, the phase transitions and ferroelectric domain structures of KNN thin films are not well understood. In this work, we employ three-dimensional (3D) phase-field simulations to obtain the ferroelectric domain structures of KNN (0 ≤ x ≤ 0.5) thin films under a range of temperatures (0 K to 1300 K) and equiaxial misfit strains (–1.5% to 1.5%), based on which we establish the misfit strain-temperature phase diagrams of KNbO3 and K0.5Na0.5NbO3 thin films. We identify a wide variety of complex domain structures with coexisting ferroelectric phases, implying enhanced dielectric and piezoelectric properties. We expect this work to provide guidance for the strain engineering of domain structures and properties of KNN thin films.

The tunable ferroelectricity and piezoelectricity of the KNN piezoceramics by Na concentrations: First-principles calculations

The ferroelectric and piezoelectric properties of orthorhombic K1-xNaxNbO3 (KNN) piezoceramics with various Na concentrations x are studied by the first-principles calculations. The spontaneous polarization of KNN single crystals increases monotonically with Na concentrations, indicating an enhanced ferroelectricity in KNN. Moreover, the averaged shear, transverse and longitudinal piezoelectric coefficient d-15*, d-31* and d-33* of KNN piezoceramics are calculated from the data of KNN single crystals to show that piezoelectricity of KNN piezoceramics positively correlates with Na concentrations and exhibits the best performance at x = 0.5625, which agrees well with the available experimental data. At this concentration, d-15*, d-31* and d-33* of orthorhombic KNN piezoceramics reach maximum, which are 1.13, 1.79 and 0.85 times larger than those of KNbO3 piezoceramics, respectively. This work provides a better understanding to the effect of Na concentrations on the enhanced ferroelectricity and piezoelectricity in the KNN piezoceramics.

First-principles calculations of electrical properties, structure, and phase transition of K1-xNaxNbO3 solid solutions.

The structure, total energy and orthorhombic as well as tetragonal electronic properties of K1-xNaxNbO3 (KNN) as a function of Na concentration were studied with first principles calculations. When the Na content of KNN was gradually increased the orthogonal phase transformation occurred, which produced an enhanced piezoelectric response of the tetragonal KNN. This result proved that the high d33 originated from the phase transition. The corresponding calculations reveal that the change of Nb-O bond length is the origin of distortion of Nb-O octahedral and phase transition. In addition, the calculations observed an unusual high peak of the KNN piezoelectric parameter, which showed the same trend as the experimental results.

Synthesis of potassium sodium niobate nanostructures by hydrothermal combining with the sol-gel method

In this paper the K1−xNaxNbO3(KNN) nanostructures were synthesized by hydrothermal method using KNN gel powders as precursors. KNbO3-type orthorhombic KNN nanowires and perovskite KNN microfingers with a morphotropic phase boundary (MPB) between rhombohedral and tetragonal characterized by X-ray diffraction and Raman spectroscopy were obtained at 190°C and 220°C, respectively. KNbO3-type orthorhombic KNN nanowires had rectangular shape and the growth direction of these nanowires was [001]. The rhombohedral-tetragonal KNN microfingers were metastable, and changed the rhombohedral-tetragonal phase into the orthorhombic phase via thermal treatment at 600°C then cooled down to room temperature. Sodium dodecyl sulfate (SDS) as surfactant was added to the hydrothermal reaction. It was found that SDS could improve the crystallinity of the rhombohedral-tetragonal K0.52Na0.48NbO3 and reduce the impurity effectively. The tetragonal-cubic phase transition temperature (Tc) of the rhombohedral-tetragonal powders appeared at 555°C.

Top-Seeded Solution Growth and Properties of K1–xNaxNbO3 Crystals

A series of large-sized (maximum 16 × 16 × 20 mm3), high-quality K1–xNaxNbO3 (x = 0.118, 0.378, 0.462, 0.545, and 0.666) single crystals were successfully cultivated using the top-seeded solution growth method. The crystallization and structures of the K1–xNaxNbO3 single crystals were studied using first-principles calculations and X-ray diffraction, respectively. The segregation of the K1–xNaxNbO3 single crystals was investigated, which enabled precise control of the individual components of the crystals during growth. Excellent properties were obtained without annealing, including a low dielectric loss (minimum 0.2%), a saturated hysteresis loop with a low coercive field Ec, a large piezoelectric coefficient d33 (d33 = 161 pC/N when x = 0.378), and a low leakage current density J (10–6 A/cm2). These results indicated that the K1–xNaxNbO3 (x = 0.118, 0.378, 0.462, 0.545, and 0.666) crystals can be used as a high-quality, lead-free piezoelectric material.

Structure, Phase Transition, and Electronic Properties of K1−xNaxNbO3 Solid Solutions from First‐Principles Theory

With extensive first‐principles calculations, we investigated the geometric structure, phase transition, and electronic properties of orthorhombic, monoclinic, and tetragonal K1−xNaxNbO3 (KNN) as functions of the Na content. We found that KNN undergoes an orthorhombic‐to‐monoclinic‐to‐orthorhombic phase transition when the Na content is gradually increased. We also found that the polarization vector of the monoclinic phase can be rotated more easily than those of the orthorhombic and tetragonal phases, giving rise to an enhanced piezoelectric response of the monoclinic KNN. Furthermore, our calculations provide an interpretation for the experimentally observed unusual broad peak of the KNN piezoelectric parameters.

Piezoelectric properties and thermal stability of (Na0.53K0.47−xAgx)Nb1−xSbxO3 ceramics

AbstractMany (K1−xNax)NbO3 (KNN)‐based ceramics with high piezoelectric performance exhibit undesirable strong temperature dependence due to the orthorhombic–tetragonal polymorphic phase transition near room temperature. In order to improve the temperature stability of the ceramics, many additives have been added into the KNN‐based ceramics to shift TO–T down to below room temperature. Contrary to the previous approach (Na0.53K0.47−xAgx)Nb1−xSbxO3 (NKANS) ceramics with TO–T well above room temperature have been prepared by a conventional solid‐state reaction method. The density and the electrical properties are effectively improved by the addition of AgSbO3, and optimum piezoelectric properties are found in the ceramics with 0.05 ≤ x ≤ 0.07, with maximum kp ∼ 0.46 for NKANS5 and maximum d33 ∼ 199 pC/N for NKANS7. More importantly, kp remains virtually almost unchanged up to the TO–T temperature (≥100 °C), indicating that the NKANS ceramics exhibit a much improved piezoelectric thermal stability. The analyses suggest that both the high TO–T value and diffuse orthorhombic–tetragonal phase transition should be responsible for the good temperature stability.

Synthesis of anisometric KNbO3and K0.5Na0.5NbO3single crystals by chemical conversion of non-perovskite templates

KNbO3 (KN) and K1−xNaxNbO3 (KNN) anisometric crystals have been produced by a chemical conversion route. Crystals of the compounds K4Nb6O17, K2Nb4O11 and KNb3O8 with high aspect ratio were first grown in molten KCl. The non-perovskite compounds were further converted to KN or KNN perovskites by reaction with K2CO3 or Na2CO3 at 800 °C with or without the presence of molten KCl. Through a dissolution–precipitation mechanism needle- or plate-like KNN and KN particles with high aspect ratios were formed with preferred orientation along the [011] or [100] directions. A short reaction time was observed to yield complete chemical conversion to polycrystalline KN/KNN, while single crystals with the morphology resembling the non-perovskite templates were obtained after longer reaction time by Ostwald ripening.

Synthesis, growth mechanism and optical properties of (K,Na)NbO3 nanostructures

(K1−xNax)NbO3 (KNN) nanomaterials with various morphologies, including nanorods, step-like structures etc. were synthesized through a hydrothermal route. For KNN nanomaterials, the phase structures changed from orthorhombic to monoclinic as the x value increased from 0.27 to 0.66. Oriented attachment and aggregation are the major formation mechanism of the nanorods and the step-like structures, respectively. The compositional change in KNN induced a series of shifts of the Raman bands, of which the mechanisms are briefly discussed. A UV/vis absorption study shows that, for K0.50Na0.50NbO3 nanorods, two interband transitions from O2p electron states to empty Nb4d electron states and two intraband transitions in relation to Nb5+ cations are revealed, and the band gap is about 3.09 eV.