(K,Na)NbO3 lead-free piezoelectric ceramics synthesized from hydrothermal powders

Among various lead-free piezoelectric materials, (K,Na)NbO₃ is a very promising candidate. In this study, (K,Na)NbO₃ ceramics were sintered from mixed KNbO₃ and NaNbO₃ powders prepared by hydrothermal reaction. These two powders were mixed with distilled water in a KNbO₃/NaNbO₃ molar ratio of 1. After sintering the mixed powder, the solid solution of (Na,K)NbO₃ ceramics was obtained. The electrical properties such as the electromechanical coupling factors k_p and k₃₃, the mechanical quality factor, Q_m, and the piezoelectric constant d₃₃ of the sintered (K,Na)NbO₃ ceramics were 0.32, 0.48, 71 (radial mode), 118 ((33)mode), and 107 pC/N, respectively.     

Preparation of complex oxide thin films under hydrothermal and hydrothermal-electrochemical conditions

Thin-film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal-electrochemical methods. Hydrothermal-electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100° to 200°C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6% to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. AZrO₃ (A = Ba, Sr) thin films were also prepared on Zr metal substrates by the hydrothermal-electrochemical method. By applying a potential above ca. +2 V vs. Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. KMO₃ (M = Ta, Nb) thin films were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300°C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations. Morphotropic phase changes were also revealed in the hydrothermal system KTaO₃-KNbO₃.     

Dielectric and piezoelectric properties of alkaline-earth titanate doped (K0.5Na0.5)NbO3 ceramics

(K_0.5Na_0.5)NbO₃ (KNN) and 0.995(K_0.5Na_0.5)NbO₃-0.005AETiO₃ (AE = Mg, Ca, Sr, Ba) were successfully prepared by conventional ceramic processing and without the cold-isostatic-pressing (CIP) process. The effects of low AETiO₃ (AET) concentration on crystal structure, density, dielectric and piezoelectric properties of the KNN based ceramics were evaluated. The results show that adding MgTiO₃(MT) and BaTiO₃(BT) to KNN can lead to the appearance of a trace amount of second phase(s), reduced density and deteriorated electrical properties. Adding CaTiO₃(CT) and SrTiO₃(ST) to KNN can promote densification and optimize electrical properties. Two phase transitions at T_t-o ( the temperature at which the phase transition from orthorhombic to tetragonal occurs) and T_c (the Curie temperature) were observed in KNN and all KNN-AET ceramics, by using differential scanning calorimetry (DSC) analysis and dielectric characterization. Adding AET to KNN caused the variations of T_t-o and T_c.     

Synthesis of anisotropic NaNbO<Subscript>3</Subscript> seed crystals and fabrication of textured (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-based ceramics

High aspect ratio patelike NaNbO₃ particles with pure perovskite structure have been successfully synthesized by topochemical microcrystal conversion (TMC) from plate-like precursor particles of the layer-structured Bi_2.5Na_3.5Nb₅O₁₈. By changing the Bi_2.5Na_3.5Nb₅O₁₈/Na₂CO₃ ratio, large and thin NaNbO₃ particles with a thickness of approximately 0.5 μm and a width of approximately 20 μm were obtained. The obtained NaNbO₃ particles is quite suitable for fabricating textured (K_0.5Na_0.5)NbO₃-based ceramics. Using the fine platelike NaNbO₃ particles as templates, dense <001> -oriented (K_0.5Na_0.5)NbO₃-0.5 mol %MnO₂ ceramics with high texture quality (Lotgering factor F ₀₀₁ = 87 %) and excellent piezoelectric properties were produced by templated grain growth. Compared with randomly oriented ceramics, textured samples show greatly enhanced properties. The room-temperature strain S, the piezoelectric coefficient d ₃₃ ^* and d ₃₃ reach up to 0.093 %, 233 pm/V and 195pC/N, respectively, which are all about 1.5 times larger than those of non-textured ceramics.     

Photoluminescence and electrical properties of Er-doped (K0.5Na0.5)NbO3-based transparent ceramics

Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02Ba(Bi_0.5Nb_0.5)O₃ transparent fluorescent ceramics were prepared using traditional solid-phase method. The (K_0.5Na_0.5)NbO₃ (KNN) ceramics were modified by introducing the second group elements Ba(Bi_0.5Nb_0.5)O₃ and rare earth ions Er³⁺. The effects of Er³⁺ on the structure, optical and electrical properties of transparent ceramics (K_0.5Na_0.5)NbO₃-Ba(Bi_0.5Nb_0.5)O₃ were investigated. The ceramics form a single perovskite structure and have a pseudo-cubic phase structure. Nanoscale grain size was obtained for ceramics, and the smallest average grain size is 80 nm. The ceramics have high transmittance. The ceramic 0.1 mol% Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02 Ba(Bi_0.5Nb_0.5)O₃ achieved the highest transmittance for this system with 62% and 52% at near-infrared light (1000 nm) and visible light (700 nm), respectively. The ceramics have up-conversion luminescence properties and also maintain good electrical properties. Under 980 nm excitation, the samples showed two green emission bands (518–536 nm, 536–557 nm) and one red emission band (646–677 nm). In addition, the ceramics have relaxation ferroelectricity, and a high dielectric constant. These functional ceramics with multiple properties will have greater research significance and application value.

     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

Dielectric and ferroelectric properties of (1-x)(Na0.5K0.5)NbO3-xBaTiO3 ceramics

(1-x)(Na_0.5K_0.5)NbO₃-xBaTiO₃ ceramics were prepared by a solid state reaction approach, and their dielectric and ferroelectric properties were evaluated together with the crystal structure. Three phase transitions at T_t1, T_t2 and T_t3 were observed by the combination of DTA analysis and dielectric characterization. These phase transitions corresponded to those of (Na_0.5K_0.5)NbO₃, and they were greatly pulled down by forming solid solution with BaTiO₃. The phase transition around T_t1 was incompletely diffusive and the appearance of diffusiveness of non ferro-paraelectric phase transition was an exception. The hysteresis loops changed their shapes from “square” into “thin square” with increasing x.     

Dielectric and piezoelectric properties of LiSbO3 doped 0.995 K0.5Na0.5NbO3-0.005BiFeO3 piezoelectric ceramics

LiSbO₃ doped and undoped 0.995 K_0.5Na_0.5NbO₃-0.005BiFeO₃ piezoelectric ceramics with high properties have been fabricated in air by the conventional ceramic processing. By adding LiSbO₃ to K_0.5Na_0.5NbO₃-BiFeO₃ ceramics, the dielectric and piezoelectric properties evidently increase. The doped ceramics exhibit good electrical properties. The enhanced piezoelectric properties of the ceramics should be attributed to optimum LiSbO₃ substitution and better microstructure with high density. Results show that LiSbO₃ doped K_0.5Na_0.5NbO₃-BiFeO₃ lead-free piezoelectric ceramics are a promising lead-free piezoelectric material for applications in different devices.     

Phase transition and piezoelectric properties of (1 − x)K0.5Na0.5NbO3–xLiSbO3 ceramics by hydrothermal powders

KNbO₃, NaNbO₃ and LiSbO₃ powders were synthesized by a hydrothermal route have been used to prepare (1 ? x)K_0.5Na_0.5NbO₃–xLiSbO₃ (KNN–LS; x = 0.00–0.08) ceramics. The effects of LiSbO₃ doping on the structures of KNN–LS ceramics have been systematically investigated by X-ray diffraction (XRD) and Rietveld refined XRD patterns. A gradual phase transition from orthogonal to tetragonal with the increase of LiSbO₃ content is demonstrated. Thereinto, the monoclinic phase is identified for the KNN–LS ceramic with the LiSbO₃ content of x = 0.08. Meanwhile, the XRD pattern reveals that the intensity ratio of (200)/(002) crystal face of the ceramic with x = 0.08 was bigger than one, which is different from the tetragonal phase. The tetragonal phase is revealed in the KNN–LS ceramic in the vicinity of x = 0.07, accompanying with relatively higher piezoelectric and ferroelectric properties. Tetragonal phase is beneficial to improve the piezoelectric properties of the KNN–LS ceramics.     

Characterization of (K0.5Na0.5)NbO3 powders and ceramics prepared by a novel hybrid method of sol–gel and ultrasonic atomization

(K_0.5Na_0.5)NbO₃ powders and ceramics were prepared by a novel hybrid method of sol–gel and ultrasonic atomization, in which Nb₂O₅ was used as the niobium source to replace those expensive soluble niobium salts. X-ray diffraction and thermal analysis were performed to investigate the synthesis process and phase transformation behavior of (K_0.5Na_0.5)NbO₃ powders. The results showed that (K_0.5Na_0.5)NbO₃ powders with a reasonably fine particle size and single-phase perovskite structure were formed at a temperature as low as 650 °C. Dense (K_0.5Na_0.5)NbO₃ ceramics with a relative density of 93% were obtained using the refined powders. The (K_0.5Na_0.5)NbO₃ ceramics prepared by the novel hybrid method exhibited relatively good properties (d₃₃ = 90 pC/N, k_p = 0.32, P_r = 20.6 μC/cm², T_c = 405 °C, ε_r = 712), suggesting that this novel hybrid method might be a promising method for the powders and ceramics preparation.     

Effects of K4CuNb8O23 on the structure and electrical properties of lead-free 0.94(Na0.5K0.5)NbO3–0.06LiNbO3 ceramics

Dense K₄CuNb₈O₂₃-modified (K_0.5Na_0.5)_0.94Li_0.06NbO₃ ceramics were prepared by normal sintering. The effects of K₄CuNb₈O₂₃ on the phase structure, microstructure and electrical properties of the ceramics were studied. Results showed that K₄CuNb₈O₂₃ induced a perovskite structure transition from coexistence of orthorhombic and tetragonal phases to orthorhombic symmetry. The addition of K₄CuNb₈O₂₃ promoted the sintering of (K_0.5Na_0.5)_0.94Li_0.06NbO₃ ceramics and simultaneously caused the grain growth. Moreover, K₄CuNb₈O₂₃-doping changed the (K_0.5Na_0.5)_0.94Li_0.06NbO₃ to “hard” ceramics and significantly enhanced the mechanical quality factor Q_m. It was found that the (K_0.5Na_0.5)_0.94Li_0.06NbO₃ ceramics doped with 0.60 mol% K₄CuNb₈O₂₃ exhibited a high mechanical quality factor (Q_m  983) as well as relatively large d₃₃ (136 pC/N) and k_p (35.9%), suggesting that this material is a promising candidate for lead-free piezoelectric ceramics for high-frequency applications.     

Phase transition, dielectric and piezoelectric properties of K0.5Na0.5NbO3–CaTi0.9Zr0.1O3 lead-free ceramics

Lead-free (1 − x)K_0.5Na_0.5NbO₃–xCaTi_0.9Zr_0.1O₃ + 0.75 mol%MnO₂ piezoelectric ceramics have been prepared by an ordinary sintering technique and their phase transition, dielectric and piezoelectric properties have been studied. The results of X-ray diffraction show that CaTi_0.9Zr_0.1O₃ diffuse into K_0.5Na_0.5NbO₃ lattices to form a solid solution with a perovskite structure. After the addition of CaTi_0.9Zr_0.1O₃, both the cubic–tetragonal and tetragonal–orthorhombic phase transition temperatures decrease, and a relaxor behavior is induced. Coexistence of the orthorhombic and tetragonal phases is formed in the ceramics with 0.03 < x < 0.07 at room temperature. Owing to the higher number of possible polarization states resulting from the coexistence of the two phases, the piezoelectric properties of the ceramics are enhanced significantly. The ceramic with x = 0.05 exhibits the following optimum properties: d ₃₃ = 203 pC/N, k _p = 45.0%, and T _C = 342 °C.     

Effect of CuO and MnO2 on sintering temperature, microstructure, and piezoelectric properties of 0.95(K0.5Na0.5)NbO3-0.05BaTiO3 ceramics

In this letter we report the effect of CuO and MnO₂ additives on the sintering behavior of 0.95(Na_0.5K_0.5)NbO₃-0.05BaTiO₃ ceramics. It was found that the composition corresponding to 0.95(Na_0.5K_0.5)NbO₃-0.05BaTiO₃ + 2.0 mol% CuO + 0.5 mol% MnO₂, sintered at 950 °C for 10 h, exhibited excellent piezoelectric properties corresponding to: k_p = 0.41, d₃₃ = 248 pC/N, Q_m = 305, ε₃^T/ε₀ = 1258, and T_c = 280 °C. These results indicate the prominence of this composition in lead-free systems.     

Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO₃, KNbO₃ (KNO), and NaNbO₃ (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO₃ (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.     

Characterization of ternary (Na0.5K0.5)1?xLixNbO3 lead-free piezoelectric ceramics prepared by molten salt synthesis method

(Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics were prepared by molten salt synthesis method. Pure perovskite-phase powder was obtained at a low temperature of 740 °C with a grain size of below 800 nm. The effects of the LiNbO₃ on phase transition, microstructure, electrical properties, and temperature stability were investigated. A morphotropic phase boundary was identified. The scanning electron microscopy indicated that the (Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics obtained by the molten salt synthesis method have a relatively uniform particle size and microstructure. The results indicate that these materials are promising candidates for lead-free piezoelectric ceramics for practical applications.     

Dielectric behavior of perovskite glass ceramics

The recent developments of energy storage devices are concerned with larger energy storage ability, low loss and good temperature stability. It has a great technological importance in engineering science. The dielectric materials like ceramics and glass ceramics have great interest in electronic ceramic industry due to above concern. The ceramic dielectrics are used as a capacitive element in electronic circuits. The perovskite glass ceramics have very high dielectric constant and low dielectric loss. The high dielectric constant in glass ceramics is attributed to space charge polarization. In order to produce glass ceramics of high dielectric constant, barium titanate glass ceramics is the first discovered ferroelectric perovskite. In this review article, we are summarizing the dielectric behavior of perovskite glass ceramics such as BaTiO₃, SrTiO₃, PbTiO₃, (Ba,Sr)TiO₃ and (Pb,Sr)TiO₃.     

Development of transparent single-crystalline KNbO3 thin film by LPE technique

Single-crystalline KNbO₃ thin film has been successfully formed on SrTiO₃ substrate from high-temperature K₂CO₃–Nb₂O₅ solution by the liquid phase epitaxy (LPE) technique. The growth morphology was strongly influenced by the melt composition and film growth temperature. The starting material for the film preparation was a powder mixture of K₂CO₃ and Nb₂O₅. The oxides were mixed in non-stoichiometric proportion with excess K₂CO₃ as flux. Under the optimized film growth conditions using melt compositions including K₂CO₃/Nb₂O₅=52.5/47.5, 60.0/40.0 and 65.0/35.0, transparent single-crystalline KNbO₃ thin films could be obtained. The synthesized KNbO₃ thin film was subjected to precession X-ray photography in order to evaluate the crystallographic relationship with SrTiO₃ substrate, and the result was compared with a simulated diffraction pattern. The precession X-ray photography clearly indicated that the [010]_KNbO3 is not placed on the same diffraction line as [010]_SrTiO3 but is slightly shifted with a difference in angle of approximately 3°, while the [100] and [001] agree in direction for KNbO₃ and SrTiO₃. The observed lattice parameter c of KNbO₃ film was calculated to be 4.043 Å which was slightly (1.7%) larger than 3.974 Å reported for KNbO₃ bulk crystal. In-plane rotation and elongation toward substrate normal for KNbO₃ lattice on SrTiO₃ substrate were discussed from the viewpoint of release of elastic energy accumulated by lattice mismatch on the substrate.     

Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

Abstract

Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO₃, KNbO₃ (KNO), and NaNbO₃ (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO₃ (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.     

Energy storage properties of (1 − x)(Bi0.5Na0.5)TiO3–xKNbO3 lead-free ceramics

In this study, a simple compound (1 ? x)(Bi_0.5Na_0.5)TiO₃–xKNbO₃ (x = 0 – 0.12) lead-free bulk ceramic was developed for high electric power pulse energy storage applications. The dielectric and ferroelectric properties of the ceramics were measured. The results illustrate that the energy storage density of the ceramics is enhanced by the addition of KNbO₃. The influence of applied electric field, temperature, and fatigue on the energy storage properties of the ceramics was evaluated for the composition-optimized (Bi_0.5Na_0.5)TiO₃–0.1KNbO₃ ceramic. The results demonstrate that (Bi_0.5Na_0.5)TiO₃–0.1KNbO₃ ceramic is a promising lead-free material for high power pulse capacitor applications. The excellent energy storage properties of the (Bi_0.5Na_0.5)TiO₃–0.1KNbO₃ ceramics are ascribed to the reversible relaxor–ferroelectric phase transition induced by the electric field.