KNbO3 Research Articles

Oriented Plate-like KNbO3 Polycrystals: Topochemical Mesocrystal Conversion and Piezoelectric and Photocatalytic Responses.

KNbO3 (KN) with a perovskite structure is an outstanding representative of lead-free piezoelectric materials, and its mesocrystals have broad application prospects in the fields of catalysis, energy storage, and conversion. However, the formation conditions of KN mesocrystals reported so far are difficult owing to their high aspect ratio and excellent preferred orientation. In this study, the solvothermal process was used successfully to prepare the flake-like potassium salt of Lindquist hexaniobate (K8Nb6O19·10H2O). Subsequently, the precursor niobate was calcined to prepare two-dimensional (2D) plate-like KN mesocrystals. The formation mechanism of the plate-like KN mesocrystals is further revealed from a paired topochemical mesocrystal conversion of K8Nb6O19·10H2O niobate. Finally, the microscopic piezoelectric and photocatalytic responses of the obtained plate-like KN mesocrystals were investigated. The high piezoelectric coefficient of plate-like KN mesocrystals implies that excellent charge separation promotes the photocatalytic performance of rhodamine B (RhB). This study provides a strategy for the efficient application of 2D oriented materials in the field of piezoelectricity and photocatalysis.

Two-Step Regulation Strategy Improving Stress Transfer and Poling Efficiency Boosts Piezoelectric Performance of 0-3 Piezocomposites.

The rapid development of flexible micropower electronics has aided the opportunity for the broader application of flexible piezoelectric composites (PCs) but has also led to higher requirements for their power generation. Among them, 0-3 PCs with embedded zero-dimension piezoparticle fillers, although low cost and easy to prepare, suffer from suboptimal output performance because of inherent structural defects. In this work, the voltage output was increased from 3.4 to 12.7 V under a force of 7 N, through first-step regulation by aligning the KNbO3 (KN) particles in the polydimethylsiloxane (PDMS) matrix; then, a significantly enhanced current output (from 0.7 to 4.5 μA) through second-step regulation by introducing copper nanorods (Cu NRs) interspersed in the gaps between the KN chains. Consequently, the proposed PC exhibits much higher power density, 37.3 μW/cm2, than that of random KN/PDMS and thus shows good potential for high-performance, flexible piezoelectric energy harvesters.

Insight for the construction of R-T phase boundary in KNN piezoceramics from the view of energy band structure and electron density

The crystal structures and associated energy band structures of classic lead-based PZT (PbZr0.5Ti0.5O3) and lead-free KNN (K0.5Na0.5NbO3) piezoelectric ceramics prepared by a solid-state method are systematically studied based on the First-principles calculations. For better understanding relations between energy band structures and the phase structures in the PbZr0.5Ti0.5O3 and KNN, the components PT (PbTiO3)/PZ (PbZrO3) and KN (KNbO3)/NN (NaNbO3) are also calculated. The results suggest that the vital contribution to R-T morphotropic phase boundary (MPB) in PbZr0.5Ti0.5O3 comes from the hybridization of A-site and B-site ions, which causes stretching and tilting of oxygen octahedron and therefore leads to the formation of R and T phases. The KNN behaves differently. Moreover, it's concluded that the band structure highly inherited from tetragonal PbTiO3 plays an important role in the construction of the R-T phase boundary according to the comparison of PbZr0.5Ti0.5O3 and KNN. In general, the work inspires an idea from the view of energy band structure to realize R-T phase coexistence as well as improved piezoelectric properties in KNN-based lead-free piezoceramics.

Topotactic Growth of Free-Standing Two-Dimensional Perovskite Niobates with Low Symmetry Phase

Here, we report a novel topotactic method to grow 2D free-standing perovskite using KNbO3 (KN) as a model system. Perovskite KN with monoclinic phase, distorted by as large as ∼6 degrees compared with orthorhombic KN, is obtained from 2D KNbO2 after oxygen-assisted annealing at relatively low temperature (530 °C). Piezoresponse force microscopy (PFM) measurements confirm that the 2D KN sheets show strong spontaneous polarization (Ps) along [101̅]pc direction and a weak in-plane polarization, which is consistent with theoretical predictions. Thickness-dependent stripe domains, with increased surface displacement and PFM phase changes, are observed along the monoclinic tilt direction, indicating the preserved strain in KN induces the variation of nanoscale ferroelectric properties. 2D perovskite KN with low symmetry phase stable at room temperature will provide new opportunities in the exploration of nanoscale information storage devices and better understanding of ferroelectric/ferroelastic phenomena in 2D perovskite oxides.

Solvothermal Reaction and Piezoelectric Response of Oriented KNbO3 Polycrystals.

KNbO3 (KN) piezoelectric polycrystals were prepared by a two-step solvothermal reaction process with the managed organic solvents as reaction mediums at a low temperature for a short time. In the solvothermal reaction system, the formation mechanism of polycrystalline KN is mainly the dissolution-deposition mechanism. The influences of alkalinity, viscosity, and the polarity for reaction mediums on the formation of the niobates were investigated. The chemical reaction mechanisms of niobate products and formation mechanism of niobate crystals from the precursor were clarified. The regulating and controlling mechanism of the phase compositions, the morphologies, and the lattice constants for the niobates obtained in varied reaction mediums were revealed. The obtained KN piezoelectric polycrystals are constructed from oriented KN nanocrystals. Piezoelectric hysteresis loops of cuboid KN polycrystals were detected for the first time. A prepared cuboid KN polycrystal shows an average d33* value of 32 pm/V. The study provides a strategy for the development of oriented KN piezoelectric materials to apply the orientation engineering.

Crystal Structure, Phase Transitions and Photoferroelectric Properties of KNbO3-Based Lead-Free Ferroelectric Ceramics: A Brief Review

Ferroelectric KNbO3 (KN) ceramics were first fabricated in the 1950s, however, their use in commercial technical applications has been hampered by inherently challenging processing difficulties. In the early 1990s, the interest in KN ceramics was revived by the pursuit of Pb-free piezoceramics. More recently the search for inexpensive photovoltaic materials alternative to Si prompted bandgap engineering studies in KN-based solid solutions. If the ferroelectric and piezoelectric properties of KN-based ceramics are now well established, the understanding of chemical doping on the bandgap of KN-based ceramics is still in its infancy. Here we provide a brief review on the current understanding of the structure-property relationships in this class of materials, which successively covers crystal structures, structural phase transitions, lattice dynamics, polarization, solid solutions and bandgap engineering of KN.

High piezoelectric properties above 150 °C in (Bi0.5Na0.5)TiO3-Based lead-free piezoelectric ceramics

(Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoelectric ceramics exhibit excellent piezoelectric properties, but its piezoelectric property with high thermal stability is the major obstacle limiting the potential application. In this study, the KNbO3 (KN) was adopted to 0.875(Bi0.5Na0.5)TiO3-0.125BaTiO3 (BNT-12.5BT) by conventional solid-state reaction to enhance the piezoelectric properties in high temperature. The results of X-ray diffraction (XRD) indicate that KN has diffused into BNT-12.5BT lattices to form a pure perovskite structure and not exert a severe disturbance on the ferroelectric tetragonal lattice. The depolarization temperature Td is maintained at a high level. For piezoelectric properties, at KN modification x = 0.01, the piezoelectric coefficient (d33) increases to 147 pC/N from 135 pC/N for unmodified composition. Meanwhile, the d33 will increase to 187 pC/N until the depolarization temperature (185 °C) arrives, showing the excellent temperature stability of the piezoelectric properties to meet the actual demand.

High Performance Flexible Piezocomposites Based on a Particle Alignment Strategy

Piezocomposites composed of piezoelectric inorganic particles dispersed in a flexible polymer have emerged as promising materials for flexible energy harvesters and sensors. In this work, to overcome the inefficient stress transfer capability in flexible piezocomposites for energy harvester, KNbO3 (KN) particles prepared by conventional solid oxide process were aligned to form chain‐distributed particles in polydimethylsiloxane (PDMS) matrix by dielectrophoresis. The composite with 8 vol.‐% KN has peak open circuit voltage of 6.3 V and short‐circuit current of 0.82 µA under an applied force of 7 N, which are significantly improved in compared with the sample with piezoelectric particles randomly dispersed. Hugely enhanced stress‐transfer capability of the aligned composite plays a key role in achieving large piezoelectric response. This work provides a promising material candidate for high performance flexible energy harvester.

Effects of K4CuNb8O23 and GeO2 addition on microstructure and piezoelectric properties in KNbO3 ceramics

A combined additives study of lead-free KNbO3 (KN) ceramics in the solid-state reaction was performed to improve the mechanical quality factor (Qm) for high power piezoelectric applications. Improved Qm values of 1137 and 1241 were confirmed in KN specimens with addition of 1.0 mol% K4CuNb8O23 (KCN) and 1.0 mol% KCN + 0.5 wt% GeO2, respectively. It is important for achievement of the higher Qm in KN systems to control the composition of the Cu segregated phase. In this study, the Cu segregated phase which was necessary to improve the Qm resulted from decomposition of the KCN compound during the sintering. Co-doping of KCN and GeO2 to KN specimens has a remarkable effect in suppressing the deliquescence accompanying KCN addition and improvement of the Qm without significant deterioration of the electromechanical coupling factor k31.

Multi-functional perovskites \u2013 an investigation of compositional and processing influence on microstructure, dielectric and ferroelectric properties

Ba(Ni0.5Nb0.5)O2.75 (BNNO) doped KNbO3 (KN) and (K0.5Na0.5)NbO3 (KNN), abbreviated as KBNNO and KNBNNO, respectively, have been recently reported to co-exhibit narrow band gaps (visible range) and strong piezoelectric/pyroelectric effects simultaneously within the same material. This had never been achieved in a single ceramic material. Such a breakthrough may allure the development of fundamentally novel multi-source energy harvesters based on only one piece of material as well as advanced optoelectronic devices with multiple functions. It has been found that the window of getting the unique combination of these properties is very narrow. Even a slight shifting away from the stoichiometry of the compositions may induce a significant loss of the properties. The reasons are expected to be in the compositions and microstructure of these materials. However, detailed information – e.g. the correlation of the compositions, processing conditions, microstructure and properties – remains to be investigated for such novel materials. In this paper, the inter-influence of different doping amounts of BNNO, calcination and sintering temperatures, phase structures and defects (potassium loss and oxygen vacancy) on the dielectric and ferroelectric properties are studied. The paper reveals the principles and provides guidance to achieving good ferroelectric properties in these emerging perovskite structured materials.

Effect of methyl alcohol and nitric acid as an etchant on the motions of ferroelectric domains in Al-doped KNbO3 single crystal

ABSTRACT The domain structure of the single crystal KNbO3 (KN) in the orthorhombic ferroelectrics phase at room temperature is investigated using optical and trinocular microscope. The cleavage face of ferroelectric Al-doped KN crystals was chemically etched using CH3OH (Methanol). At room temperature and before etching, the 60°, 90° domains were observed on the crystal surface. A dislocation model is proposed to explain the nucleation and evaporation of the domain wall under the influence of the electric field, which can be used to study the changes in motion of ferroelectric domains. After etching with HNO3 (Nitric acid) and applying an electric field, the 180° domains were observed.

Synaptic Plasticity and Metaplasticity of Biological Synapse Realized in a KNbO3 Memristor for Application to Artificial Synapse.

Amorphous KNbO3 (KN) films were grown on a TiN/SiO2/Si substrate to synthesize a KN memristor as a potential artificial synapse. The Pt/KN/TiN memristor exhibited typical and reliable bipolar switching behavior with multiple resistance levels. It also showed the transmission properties of a biological synapse, with a good conductance modulation linearity. Moreover, the KN memristor can emulate various biological synaptic plasticity characteristics including short-term plasticity, long-term plasticity, spike-rate dependent plasticity, paired-pulse facilitation, and post-tetanic potentiation by controlling the number and rate of the potentiation spike. Spike-timing-dependent plasticity (STDP), which is an essential property of biological synapses, is also realized in the KN memristor. The synaptic plasticity of the KN memristor can be explained by oxygen vacancy movement and oxygen vacancy filaments. The metaplasticity of biological synapses was also implemented in the KN memristor, including the metaplasticity of long-term potentiation and depression, and of STDP. Therefore, the KN memristor could be used as an artificial synapse in neuromorphic computing systems.

Structural and morphological evolution of an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process

A meaningful KNbO3 (KN) mesocrystal with octahedral morphology was formed using classical crystallizations including stacking and topotactic structural conversion processes.

Low-Temperature-Grown KNbO3 Thin Films and Their Application to Piezoelectric Nanogenerators and Self-Powered ReRAM Device.

Amorphous KNbO3 (KN) film containing KN nanocrystals was grown on TiN/SiO2/Si substrate at 350 °C. This KN film showed a dielectric constant (εr) and a piezoelectric strain constant (d33) of 43 and 80 pm/V at 10 V, respectively, owing to the existence of KN nanocrystals. Piezoelectric nanogenerators (PNGs) were fabricated using KN films grown on the TiN/polyimide/poly(ethylene terephthalate) substrates. The PNG fabricated with the KN film grown at 350 °C showed an open-circuit output voltage of 2.5 V and a short-circuit current of 70 nA. The KN film grown at 350 °C exhibited a bipolar resistive switching behavior with good reliability characteristics that can be explained by the formation and rupture of the oxygen vacancy filaments. The KN resistive random access memory device powered by the KN PNG also showed promising resistive switching behavior. Moreover, the KN film shows good biocompatibility. Therefore, the KN film can be used for self-powered biomedical devices.

Understanding the Structural and Electronic Effect of Zr4+-Doped KNb(Zr)O3 Perovskite for Enhanced Photoactivity: A Combined Experimental and Computational Study

Herein, for the first time we report the synthesis of a series of compositionally tunable Zr-doped novel KNb(Zr)O3 perovskites using green and facile methodology and their superior photocatalytic activities toward photoinduced hydrogen production and dye degradation. The substitutional doping of Zr(IV) in place of Nb(V) leads to a decrease in the electronic band gap by inclusion of a new acceptor level near the valence band. The results on the optimally doped perovskite for the photocatalytic hydrogen evolution reaction reveal that the 20 mol % Zr-doped KNb(Zr)O3 is 13 times more efficient as compared to the pristine KNbO3 in terms of rate of H2 evolution. The same nanocomposite is shown to exhibit 12-fold greater photocatalytic efficiency for degradation of Rhodamine B (RhB) (up to 83% after 210 min) than pure KNbO3. Density functional theory calculations are carried out to understand the Zr doping effect on the electronic structure as well as the surface hydrogen evolution reaction. Overall, an optimize...

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

0.93Bi0.5Na0.5TiO3-0.07BaTiO3 (BNTBT) and KNbO3 (KN) powders with average particle size of ∼50nm and ∼300nm 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 1.72J/cm3 under 16.8kV/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.

Structural and electrical properties of KNbO3 thin film grown on a Pt/Ti/SiO2/Si substrate using the RF magnetron sputtering method

A homogeneous KNbO3 (KN) thin film was grown on a Pt/Ti/SiO2/Si substrate using the RF sputtering method. A K2.88Nb5O15 secondary phase was formed in the KN film when the deposition and the annealing temperatures were greater than room temperature (RT) and 800 °C, respectively, owing to the evaporation of K2O. On the other hand, KNb3O8 and K3Nb5.45O15 secondary phases were formed in the KN films when the annealing temperature was less than 800 °C or the annealing time at 800 °C was shorter than 90 min. A homogeneous KN thin film was formed when it was deposited at RT and subsequently annealed at 800 °C for 90 min under the K2O atmosphere. This KN film exhibits a relative permittivity of 884 with a dissipation factor of 6.71% at 100 kHz. The leakage current density of 1.06 × 10−6 A/cm2 at 0.1 MV/cm and a breakdown field of 1.5 MV/cm were observed from this film. This film showed a saturation polarization of 21.9 μC/cm2, with a remnant polarization of 8.3 μC/cm2, and a piezoelectric strain constant of 125 pm/V.

Role of structure gradient region on dielectric properties in Ba(Zr,Ti)O3–KNbO3 nanocomposite ceramics

Crystal structures of KNbO3 (KN)/BaZrxTi1−xO3 [BZT, (0.1 ≤ x ≤ 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.

Effect of CuO on the ferroelectric and piezoelectric properties of lead-free KNbO3 ceramics

CuO assisted the densification of KNbO3 (KN) ceramics sintered at 960°C through the formation of a CuO-related liquid phase. Most of the Cu2+ ions entered the Nb5+ sites of the KN ceramics and acted as a hardener. In addition, the CuO-added KN (CKN) ceramics exhibited a double polarization versus electric field (P-E) loop and a sprout-shaped strain versus electric field (S-E) loop. The formation of these was attributed to the presence of defect dipoles formed by acceptor ions (Cu2+) and oxygen vacancies. In fact, owing to the high orthorhombic-to-tetragonal transition temperature of the CKN ceramics, the double P-E and sprout-shaped S-E loops developed without an aging process. Furthermore, the 1.0mol% CuO-added KN ceramic exhibited a very high Qm value of 3005 with a large unipolar strain of 0.15% at 8.0kV/mm.

Characterization of KNbO3 nanoplates synthesized by a stirred hydrothermal process

Lead free piezoelectric KNbO3 (KN) nanoplates were successfully synthesized through reaction between Nb2O5 and KOH at 220°C and 10M KOH during 8h. The influence of stirring hydrothermal process on the morphology was studied. X-ray patterns confirm the formation of a pure compound with a significant increase in lattice parameters relative to the bibliography. FTIR and TG measurements show the presence of hydroxyl groups located at the surface and in the bulk sample. Transmission electron microscopy observation highlights the formation of superimposed plates of KNbO3. The thickness of the plates corresponds to about twenty cells of orthorhombic KN. The measured dielectric constant εr and tangδ are 540 and 0.03, respectively at 1kHz and room temperature. A relaxation phenomenon of domains walls has been observed below 150K in the losses curves and between 450 and 530K for the permittivity curves.