KNbO3 Powders Research Articles

Microstructure Development in -Oriented (K,Na)NbO3-based Ceramics

The development of grain orientation and the factors controlling the degree of orientation in (K,Na)NbO3-based ceramics were examined using two types of platelike NaNbO3 template powders and four types of matrix powders—KNbO3, NaNbO3, and (K0.5Na0.5)NbO3 powders, and a mixture of KNbO3 and NaNbO3 powders. The NaNbO3 template powders were prepared by single- and two-step molten salt synthesis (MSS). In the ceramic specimens, the template NaNbO3 grains changed to (K,Na)NbO3. The direction of material transport in this reaction determined the degree of orientation. The sintering characteristics of the matrix powders also contributed to the control of the degree of orientation.

Synthesis and Morphology of KNbO3 Powders via Hydrothermal Method

In this research, potassium niobate (KNbO3) powder was synthesized successfully for 6–24 h in 15 M of KOH solutions at 120–200°C via the hydrothermal method. The effects of reaction temperatures and reaction times on the precipitation of KNbO3 powders were investigated. A hydrothermal precursor yielded a pure single phase KNbO3 for 18 h at 150°C. When the reaction time increased to 24 h, the morphology of KNbO3 changed from an irregular shape to a rod- or bar-shape. The reaction temperature had a marked effect on both structure and phase formation, while the reaction time could affect morphology. The direct synthesis of KNbO3 from Nb2O5 and aqueous KOH under hydrothermal conditions therefore represents an easy method for the preparation of this compound.

Piezoelectric Properties of Li-Doped (K0.48Na0.52)NbO3 Ceramics Synthesized Using Hydrothermally-Derived KNbO3 and NaNbO3 Fine Powders

[Lix(Na0.52K0.48)1-x]NbO3 (0 ≤x ≤0.091) ceramics were synthesized using hydrothermal powders and the lithium doping content was controlled to optimize their piezoelectric properties. The raw KNbO3 and NaNbO3 powders were obtained separately by a hydrothermal method and LiNbO3 powders were prepared by milling a commercial LiNbO3 single crystal. These powders were mixed with ethanol at a molar ratio LiNbO3: (Na0.52K0.48)NbO3= x : 1-x. The synthesized powders were sintered at 1060–1120 °C for 2 h. We succeeded in obtaining highly dense [Lix(Na0.52K0.48)1-x]NbO3 ceramics using hydrothermal powder. The X-ray diffraction patterns revealed that the crystal phase changed from orthorhombic to tetragonal at around x = 0.06. At this morphotropic phase boundary (MPB), the c/a ratio changed from 1.016 to 1.024 and the highest piezoelectric constant was obtained with the chemical component of [Li0.065(K0.48Na0.52)0.935]NbO3. The obtained piezoelectric properties were as follows: k33 = 0.51, ε33T/ε0 = 836, c33E = 46 GPa, d33 = 203 pC/N, and Tc = 482 °C.

Sol-gel processing and characterization of potassium niobate nano-powders by an EDTA/citrate complexing method

The present research describes a modified sol-gel technique used to obtain nano-crystalline potassium niobate (KNbO3) powders by using ethylene diamine tetraacetic acid (EDTA)/citrate as a complexing agent. The metal ions chemically interact with EDTA in the precursor sol. The aging treatments lead to the formation of a precursor-polymeric gel network. The effects of the amounts of citric acid and EDTA on the stability of the precursor sol are investigated. The influence of excess K on the formation of pure-phase KNbO3 powders is also studied. The obtained gels and powders are characterized by thermogravimetric-differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that a stable precursor sol is formed when n(CA):n(Mn+) = 3:1 and n(EDTA) :n(NH4OH) = 1:3.5. The xerogel is calcined at 700–850 °C to prepare the KNbO3 nano-powder. The smallest grain size of the sample obtained at 850 °C is about 60 nm when the K/Nb molar ratio equals 1.2.

Low temperature formation of (NaxK1−x)NbO3 from hydrothermally synthesised NaNbO3

Na0·5K0·5NbO3 was synthesised by heating a mixture of KNbO3 and NaNbO3 powders to 800°C and cooling immediately. The KNbO3–NaNbO3 powder mixture was obtained after 10 h of reacting NaNbO3 in a 6M KOH solution at 200°C. This reaction caused the dissolution of NaNbO3 and precipitation of KNbO3. The dissolution–reprecipitation process led to an intimate mixture of KNbO3 and NaNbO3 powders with much smaller particle sizes and a surface area that was 3·5 times greater than the separately hydrothermally synthesised KNbO3 and NaNbO3 powders. The increased surface area, smaller grain sizes and intimate mixing of the powders were responsible for the much faster conversion to a Na0·5K0·5NbO3 solid solution powder.

Piezoelectric Materials Synthesized by the Hydrothermal Method and Their Applications.

Synthesis by the hydrothermal method has various advantages, including low reaction temperature, three-dimensional substrate availability, and automatic polarization alignment during the process. In this review, powder synthesis, the fabrication of piezoelectric thin films, and their applications are introduced. A polycrystalline lead zirconate titanate (PZT) thin film was applied to a micro ultrasonic motor, and an epitaxial lead titanate (PbTiO3) thin film was estimated as a ferroelectric data storage medium. Ferroelectric and piezoelectric properties were successfully obtained for epitaxial PbTiO3 films. As lead-free piezoelectric powders, KNbO3 and NaNbO3 powders were synthesized by the hydrothermal method and sintered together to form (K,Na)NbO3 ceramics, from which reasonable piezoelectric performance was achieved.

An Efficient Approach for the Direct Synthesis of KNbO3 Powders

Pure potassium niobate (KNbO3) powders were prepared at low temperatures by an efficient method using K2CO3 and Nb2O5 as raw materials and urea as fuel. The crystal structure and morphology of the powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that cubic phase KNbO3 powders can be obtained at 550°C by the low-temperature combustion method and with the temperature increasing to 700°C they change to orthorhombic phase KNbO3. The synthesis parameters are studied to reveal the optimized synthesis conditions for preparing pure KNbO3 powders.

An alternative approach of solid-state reaction to prepare nanocrystalline KNbO3

A simple preparation of KNbO3 powders was proposed by an alternative approach of solid-state reaction. Stoichiometric niobium oxalate and potassium acetate were mixed in water and then dried. It was demonstrated that an ion-exchange reaction occurred with the formation of K[NbO(C2O4)2]·nH2O intermediate. The single-phase KNbO3 powders were synthesized when K[NbO(C2O4)2]·nH2O intermediate was calcined between 500 and 800 °C for 3 h. KNbO3 powders obtained at 500 °C are determined as orthorhombic structure with an average particle size of 20–50 nm by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscopy (TEM) analysis. The morphologies of KNbO3 obtained at different temperatures were observed by SEM and TEM analysis. The average band gap energy is estimated to be 3.16 eV by UV–vis diffuse reflectance spectra.

Phase Evolution in Low-Dimensional Niobium Oxide Synthesized by a Topochemical Method

In this paper, we report a catalyst-free topochemical method, combined with molten salt synthesis (MSS), to synthesize, on a large scale, rodlike and platelet single crystals of Nb(2)O(5). Rodlike KNb(3)O(8) and platelet K(4)Nb(6)O(17), which were fabricated as the precursors by the molten salt method, were treated by proton exchange and heat treatment to synthesize the rodlike H-Nb(2)O(5) and platelet T-Nb(2)O(5) single crystal, respectively. The synthesized niobium pentaoxides retained the rodlike and platelet shapes of their precursors. The structural changes involved in the process were investigated by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. A possible topochemical reaction mechanism is proposed. Furthermore, rodlike and platelet KNbO(3) powders were derived from stable H-Nb(2)O(5) and T-Nb(2)O(5), respectively.

Synthesis of Nondoped Potassium Niobate Ceramics by Ultrasonic Assisted Hydrothermal Method

Non doped potassium niobate (KNbO3) crystal powders were synthesized by ultrasonic-assisted hydrothermal method for use in lead-free piezoelectric ceramics. By using the ultrasonic assist technique, the yield constant of the KNbO3 crystal powders became 96% with 3 h of reaction time. It was 4 times faster than that of the previous non ultrasonic assist hydrothermal method for the KNbO3 crystal powders. This result indicated that the high-intensity ultrasound radiation was effective for overcoming the serious problem of slow hydrothermal reaction. The reaction temperature was 190 °C, which was 20 °C lower than that of previous hydrothermal method. After sintering of hydrothermal KNbO3 powders at 1030 °C for 1 h, the obtained ceramics pellet was characterized by admittance measurement, and the piezoelectric performance was examined. From the admittance measurements corresponding to the radial and thickness vibration modes, the coupling factors kr and kt and the piezoelectric constants d31 and d33 were determined to be 0.17 and 0.28, and -31 and 51 pm/V, respectively.

Morphotropic Phase Boundary of Lead-Free Piezoelectric Ceramics from BNT- KN System

In this research work, the investigation of the morphotropic phase boundary, physical properties and electrical properties of lead- free piezoelectric materials of bismuth sodium titanium oxide: (Bi0.5Na0.5)TiO3 (BNT) and potassium niobium oxide : KNbO3 (KN) in the ceramic system of (1-x)BNT- xKN where x= 0.00, 0.03, 0.05, 0.08, 0.10 and 0.15 by modified mixed oxide method was carried out. The BNT and KN powders were prepared separately using calcination technique with optimum calcination temperature for producing both BNT and KN powders of about 800°C. XRD results revealed that the BNT-KN ceramics with low KN content of x less than 0.05 contain ferroelectric perovskite phase with rhombohedral symmetries while the higher KN content ceramics have mixed symmetries between rhombohedral and orthorhombic. From the evidences including phases, microstructures and dielectric data of the BNT-KN ceramics, it may be assumed that the morphotropic phase boundary may be at around the composition of x = 0.05.

Nondoped Potassium Niobate Ceramics Synthesized by Hydrothermal Method with Optimum Temperature Condition

Nondoped potassium niobate (KNbO3) ceramics were synthesized with fine powders obtained by a hydrothermal method. The hydrothermal method employs a chemical reaction in a solution. Low reaction temperature and self-assembly growth resulted in high-quality KNbO3 powders. In this study, various temperature conditions from 180 to 230 °C were examined for high-performance KNbO3 ceramics. After sintering at 1030 °C using hydrothermally crystal powders under respective conditions, the dielectric loss and permittivity of each ceramic were measured. As a result, the suitable temperature condition of the hydrothermal synthesis for piezoceramics was 210 °C. With admittance measurements, the piezoelectric performance of fabricated KNbO3 ceramics was confirmed. From the resonance frequency response corresponding to the radial vibration and thickness vibration modes, the coupling factors kr and kt and the piezoelectric constants d31 and d33 were measured to be 0.14 and 0.26, and -28 and 55 pm/V, respectively.

Preparation of KNbO<sub>3</sub> Powder by Modified Pechini Method from Layered Perovskite Aqueous Solution

KNbO3 (KN) powder was synthesized by modified Pechini method (polymeric precursor method). KN powder with different crystal structure and grain size distribution was synthesized by different drying temperature. In this paper, the mechanism of preparation of KN powder was discussed in thermal decomposition behavior. The obtained powder was characterized by XRD, TEM, FT-IR and TG-DTA.

Fabrication and Characterization of Alkoxy-Derived (Na, K)NbO<sub>3</sub> Series Powder

Various KNbO3 powders have been derived from sol-gel process by using different preparation and heating conditions. KNbO3 powder derived from ethanol solution and fabricated at 800oC was most suitable as the raw material of KNbO3 bulk ceramics, because of the small grain size of about 250 nm and its homogeneous size distribution.

Effect of Processing Parameters of KNbO3 Powder Prepared from Aqueous Solution of Layered Perovskite

Preparation of KNbO3 powder from aqueous solution of layered perovskite K2NbO3F has been investigated. The prepared powders showed a pseudo-cubic KNbO3 structure, and need heat treatment at 1000°C to convert in the crystal structure to orthorhombic ferroelectrics. In this paper, we report a modified processing for the preparation of a better organized orthorhombic KNbO3 powders by firing at lower temperature of 800°C by the introduction of Pechini method.

Processing and ferroelectric behavior of textured KSr2Nb5O15 ceramics

-textured KSr2Nb5O15 (KSN) ceramics were fabricated in a matrix of SrNb2O6 and KNbO3 powders via reactive templated grain growth (TGG), using -oriented acicular KSN particles as the template for grain growth. KSN phase formation started at 1100 °C and was complete at 1300 °C. Template particles grew at the expense of matrix grains with increasing sintering time and temperature, resulting in improved texture fraction, f. A maximum fraction of 0.98 was obtained in 1.5 wt% Nb2O5 and 10 wt% template containing samples sintered at 1450 °C for 6 h. A template content of 10 wt% resulted in a high quality of texture with a narrow distribution of elongated grains in [001]. The highly textured ceramics (f ≥ 0.9) had a remanent polarization of 0.19 C/m2 in the polar c direction, [001], as compared to 0.03 C/m2 in the non-polar a or b direction and 0.04 C/m2 as for the random sample. The estimated saturation polarization (Psat) was 0.25 C/m2 which is comparable to reported KSN single crystal value of 0.25 C/m2. This indicates that TGG is a viable method to fabricate textured ceramics having ferroelectric properties close to the single crystals.

Preparation of KNbO3 thin films onto alumina substrates by polymeric precursor method

This work presents the development of KNbO3 powders and thin films prepared by the low-cost solution chemical method based on Pechini process. The influence of many synthesis parameters, such as coating solution viscosity, deposition method (dip- or spin-coating), [K/Nb] ratio, heat treatment and effect of multilayers deposition, on the phase formation and microstructure of thin films prepared onto alumina substrates were analyzed. It was verified that using a stoichiometric coating solution, K deficient films are formed. The K amount in the coating solution was increased in order to avoid this deficiency. Single-phased KNbO3 samples were obtained at 550 °C using solutions with [K/Nb]1.25 and 1.50. These films are continuous, homogeneous and crack-free, and present a thickness about 100 nm per layer. The deposition of multilayers increases the film thickness, as well as it improves its homogeneity and density. Both deposition methods tested, spin- and dip-coating, lead to good results, after deposition parameters optimization. The best results are achieved using coating solution viscosity around 22×10−2 P, dipping and withdrawing speed of 0.16 mm s−1 for dip-coating and rotation rate around 3000 rpm during 20–40 s for spin-coating.

Formation of Niobates from Aqueous Peroxide Solution

Abstract KNbO3 and LiNbO3 powders and thin films were fabricated through heat-treatment of their peroxide precursors prepared by the reaction of aqueous hydrogen peroxide solution with metal alkoxide. In comparison with the conventional alkoxide solution method, which is not successful for KNbO3, aqueous peroxide solution method is successful for KNbO3 with the advantages such as low weight loss and easy handling in preparing the niobates.

Texture Development in KSr<sub>2</sub>Nb<sub>5</sub>O<sub>15</sub> Ceramics Fabricated by Reactive Templated Grain Growth

Texture Development in KSr<sub>2</sub>Nb<sub>5</sub>O<sub>15</sub> Ceramics Fabricated by Reactive Templated Grain Growth

Hydrothermal synthesis of KNbO3 and NaNbO3 powders

Orthorhombic KNbO3 and NaNbO3 powders were hydrothermally synthesized in KOH and NaOH solutions (6.7–15 M) at 150 and 200 °C. An intermediate hexaniobate species formed first before eventually converting to the perovskite phase. For synthesis in KOH solutions, the stability of the intermediate hexaniobate ion increased with decreasing KOH concentrations and temperatures. This led to significant variations in the induction periods and accounted for the large disparity in the mass of recovered powder for different processing parameters. It is also believed that protons were incorporated in the lattice of the as-synthesized KNbO3 powders as water molecules and hydroxyl ions.