Hydrothermal synthesis of submicron NaNbO3 powders

Nano-crystals of ferroelectric NaNbO₃ phase were prepared by hydrothermal method in one step. The influence of temperature, concentration of Nb₂O₅ and NaOH, and reaction duration on structure and morphology was analyzed. Temperature has a marked effect on phase formation, while concentration of Nb₂O₅ and NaOH can affect both structure and morphology. Lower ratio of NaOH/Nb₂O₅ facilitates formation of orthorhombic NaNbO₃. Reaction duration only plays an important role in the formation process at lower temperatures. The intermediate phases of sodium niobates may transform into NaNbO₃ by prolonging reaction duration or annealing at certain temperature.     

Preparation of Plate‐Like Sodium Niobate Particles by Hydrothermal Method

Sodium niobate NaNbO₃ hydrate (NN‐hydrate) particles with a plate‐like morphology were prepared at 140°C for 2 h in 12 mol/L of NaOH by the hydrothermal method. Bar‐like Na₈Nb₆O₁₉·13H₂O particles were synthesized at as low as 100°C for 2 h. This work demonstrates that by carefully optimizing the reaction condition, we can selectively fabricate niobate structures, including the bar‐like, plate‐like, fibers and cube particles through a direct reaction between NaOH solution and Nb₂O₅. It was found that Nb₆O₁₉^8? formed was an important premise for formation of the NN‐hydrate, and lower [OH^–] was not favorable in preparing the NN‐hydrate as there was an optimum [OH^?]. Through researching effects of the reaction temperature, time, concentration of NaOH, and content of Nb₂O₅ on the NN‐hydrate structure and evolution, the formation mechanism from solid reactants to the intermediate were investigated. After calcining at 800°C, the synthesized NN‐hydrate particles made a phase almost transform to the perovskite NaNbO₃, and the morphology of these calcined particles was still plate‐like.     

Using KNbO3 catalyst produced from a simple solid-state synthesis method in a new piezophotocatalytic ozonation hybrid process

The present work reports the development and application of potassium niobate (KNbO₃) as a catalyst in a novel hybrid piezophotocatalytic ozonation process aimed at wastewater remediation. Pure KNbO₃ samples were produced through a simple solid-state synthesis using water-soluble ammonium niobate (V) oxalate hydrate (C₄H₄NNbO₉·xH₂O) as niobium source, employing different potassium precursors (KNO₃, K₂CO₃, KOH, and C₈H₅KO₄). The synthesis was also carried out using powdered niobium oxide as a precursor, aiming to evaluate the differences between the niobates obtained. The results achieved in this study show that all the niobates produced using ammonium niobate (V) oxalate hydrate were composed solely of the orthorhombic structure of KNbO₃, while the materials synthesized using niobium oxide exhibited the rhombohedral structure of KNbO₃ along with niobium-rich potassium niobates (K₃Nb₈O₂₁, K₂Nb₄O_21, and KNb₃O₈) and residual niobium oxide. This behavior was attributed to the enhanced chemical homogeneity derived from the synthesis using ammonium niobate (V) oxalate hydrate, which facilitated the reaction between the components during the thermal treatment step. Furthermore, the optical and morphological properties of the niobates were considerably influenced by the application of different potassium salts. Owing largely to its morphological and electrical properties, the material synthesized using potassium hydrogen phthalate displayed the highest photocatalytic activity in terms of methylene blue discoloration among the niobates produced using C₄H₄NNbO₉·xH₂O. Finally, the proposed piezophotocatalytic ozonation process was found to be a highly efficient strategy for the discoloration of methylene blue, as it successfully harnessed the synergy between the multiple mechanisms involving active radical generation toward the development of a highly promising hybrid advanced oxidation process.     

Efficient degradation of methylene blue dye by catalytic oxidation using the Na8Nb6O19·13H2O/H2O2 system

Na₈Nb₆O₁₉·13H₂O particles were synthesized by a simple hydrothermal method. The catalysts were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and thermogravimetric and differential scanning (TG-DSC). The XRD and TG-DSC analyses indicated that Na₈Nb₆O₁₉·13H₂O was an intermediate hexaniobate during the preparation of NaNbO₃ powders. Methylene blue (MB) dye degradation using Na₈Nb₆O₁₉·13H₂O/H₂O₂, Nb₂O₅/H₂O₂ and NaNbO₃/H₂O₂ systems were investigated, respectively. Among the catalytic oxidation systems, Na₈Nb₆O₁₉· 13H₂O showed the highest activity for degradation of MB in the presence of H₂O₂. The results indicated that the dye degradation efficiency could be 93.5% at 30 °C after 60 min in the presence of the Na₈Nb₆O₁₉·13H₂O/H₂O₂ system. It was also found that the degradation of MB over the catalytic systems followed pseudo-first-order kinetics, and the degradation rate was 0.02376 min⁻¹ in the Na₈Nb₆O₁₉·13H₂O/H₂O₂ system, which was higher than that in the Nb₂O₅/H₂O₂ and NaNbO₃/H₂O₂ systems. A possible mechanism for MB catalytic oxidation degradation using the Na₈Nb₆O₁₉·13H₂O/H₂O₂ system was proposed.     

In situ Precursor-Template Route to Semi-Ordered NaNbO3 Nanobelt Arrays

We exploited a precursor-template route to chemically synthesize NaNbO₃ nanobelt arrays. Na₇(H₃O)Nb₆O₁₉·14H₂O nanobelt precursor was firstly prepared via a hydrothermal synthetic route using Nb foil. The aspect ratio of the precursor is controllable facilely depending on the concentration of NaOH aqueous solution. The precursor was calcined in air to yield single-crystalline monoclinic NaNbO₃ nanobelt arrays. The proposed scheme for NaNbO₃ nanobelt formation starting from Nb metal may be extended to the chemical fabrication of more niobate arrays.     

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

In recent years, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) has become popular and promising among perovskite lead‐free piezoceramic systems. In this study, the chemical stability of KNN powders in aqueous medium was investigated as a function of pH, time, and powder surface area. To better understand the dissolution behavior of the complex KNN stoichiometry, subconstituents such as potassium niobate (KNbO₃, KN) and sodium niobate (NaNbO₃, NN) were investigated separately first. Results showed that all of the cations in the structure underwent dissolution in different values. Indicating that KNN undergoes incongruent dissolution in aqueous medium, the dissolution of A site cations was higher at lower pH while the dissolution of B site cations increased at higher initial pH. The order of released cation concentrations (C_A1 = K > C_{A2 = Na} > C_B = Nb) fits with inverse relationship of cation field strength (FS) order, B = Nb⁵⁺_FS > A₂ = Na⁺_FS>A₁ = K⁺_FS, at pH 4, 7 and 10 for NN, KN, and KNN. Calculated diffuse layer thickness from the ICP data confirmed to outer amorphous layer in TEM image. Also, the ratio of normalized cation concentration versus surface area of powders showed that incongruent dissolution kinetic was driven by the diffusion step.     

Crystallization of LiNbO3 and NaNbO3 in niobiosilicate glass–ceramics

Although glass–ceramics have been widely explored for their thermal stability and mechanical properties, they also offer unique symmetry-dependent properties such as piezoelectricity and pyroelectricity through controlled crystallization of a polar phase. This work examines crystallization of LiNbO₃ in a 35SiO₂–30Nb₂O₅–35Li₂O mol% composition and crystallization of LiNbO₃ and NaNbO₃ in a 35SiO₂–30Nb₂O₅–25Li₂O–10Na₂O mol% composition. Crystallization kinetics are examined using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory where the Avrami exponent, n, is calculated to be 1.0–1.5. Microscopical analysis shows dendritic morphology, which when combined with the JMAK analysis, suggests diffusion-controlled one-dimensional growth. Adding Na₂O to the glass composition increases the inter-diffusivity of ions which causes LiNbO₃ to crystallize faster and lowers the activation energy of transformation from 1054 ± 217 kJ/mol in the ternary composition to 882 ± 212 kJ/mol. Time-temperature-transformation diagrams are presented which show that the temperature for maximum rate of transformation for LiNbO₃ is ∼650°C and for NaNbO₃ is ∼715°C.     

Filled oxygen‐deficient strontium barium niobates

Filled strontium barium niobate (SBN, Sr_xBa_6?xNb₁₀O_30?δ) compositions from the barium end‐member to a Sr:Ba ratio of 80:20 were fabricated using conventional sintering under oxygen‐deficient conditions. Single‐phase specimens were obtained when sintered at 1350°C under a reducing condition of about 1.0 × 10^?16 atm pO₂. X‐ray diffraction and energy dispersive spectroscopy using scanning electron microscopy and transmission electron microscopy were used to confirm the phase‐purity. Lattice parameters derived from Rietveld refinements were used to trace Vegard's Law over the solid solution. It was found that Vegard's Law is followed over two ranges, which is believed to be due to preferential site occupancy of the A sites as the Sr content changes. The successful fabrication of filled SBN bronzes provides an avenue for correlating trends in electrical characteristics of the SBN solid solution.     

Hydrothermal synthesis of NaNbO3 with low NaOH concentration

NaNbO₃ fine powders were prepared by reacting niobium pentoxide with low NaOH concentration solution under hydrothermal conditions at 160 °C. The reaction ruptured the corner-sharing of NbO₆ octahedra in the reactant Nb₂O₅, yielding various niobates, and the structure and composition of the niobates depended on the [OH⁻] and reaction time. The fine Nb₂O₅ powder first aggregated to large particles and then turned to metastable intermediates with multifarious morphology. The reaction was fast for the situation of [OH⁻] = 2 M. The [OH⁻] determined the structure of final products, and three types of NaNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, respectively, depending on the [OH⁻]. The low [OH⁻] was propitious to yield orthorhombic NaNbO₃. The present work demonstrated that higher [OH⁻] was not favored to synthesize NaNbO₃ powders and the conversion speed in this reaction was not in proportion to the [OH⁻].     

The influence of different niobium pentoxide precursors on the solid-state synthesis of potassium sodium niobate

Two batches of K_0.5Na_0.5NbO₃ were prepared from the orthorhombic and monoclinic Nb₂O₅ polymorphs and potassium and sodium carbonates. The influence of the different Nb₂O₅ precursors on the solid-state synthesis of K_0.5Na_0.5NbO₃ was studied. To reduce the particle size, both types of Nb₂O₅ were milled prior to use. XRD and TEM analyses showed that the milled orthorhombic Nb₂O₅ was single phase; however, after milling the monoclinic Nb₂O₅ consisted of large monoclinic particles and orthorhombic nanocrystals. The latter reacted with the carbonates to form (K_xNa_1-x)NbO₃ solid solutions with varying K/Na molar ratios, while the orthorhombic Nb₂O₅ reacted to form a homogeneous solid solution of K_0.5Na_0.5NbO₃. Sintering of the two powder compacts resulted in different densification behavior and microstructure. This study shows the important influence of the Nb₂O₅ precursor phase and the particle size distribution on the homogeneity and further densification of the potassium sodium niobate solid solution.     

Chloride flux growth of crystalline strontium niobates and nitridation to perovskite SrNbO2N

Highly crystalline (110) layered perovskite Sr₂Nb₂O₇, (111) layered perovskite Sr₅Nb₄O₁₅ and complex perovskite Sr₄Nb₂O₉ were prepared by NaCl-KCl flux growth method from SrCO₃ and Nb₂O₅. This flux synthesis achieves single strontium niobate phase in contrast to mixed niobates from the solid state reaction with the same heating parameters. A little excess of Sr source was found to be required for the synthesis of Sr₅Nb₄O₁₅ and Sr₄Nb₂O₉ at elevated temperature due to slight evaporation. The three strontium niobates were converted to perovskite SrNbO₂N via thermal ammonolysis under NH₃ flow at 900 °C. Post-wash treatment was performed to remove the byproduct SrO. This makes additional nanopores in SrNbO₂N in the cases of Sr₅Nb₄O₁₅ and Sr₄Nb₂O₉, and results in increasing surface areas of SrNbO₂N with Sr:Nb ratios in the precursors (from 9.9 to 19.8 and 35.5 m²/g). On the other hand, the UV–Vis diffusion reflectance spectra reveal decreasing light absorption by defects in SrNbO₂N in this order. This suggests fewer low-valent Nb defects in SrNbO₂N prepared from precursor with higher Sr:Nb ratio. SrNbO₂N prepared from Sr₄Nb₂O₉ would be advantageous for applications that require high surface area and low defect density of the material.     

Flux-mediated synthesis and photocatalytic activity of NaNbO3 particles

Using molten-salt synthetic techniques, NaNbO₃ (Space group Pbcm; No. 57) was prepared in high purity at a reaction time of 12 hours and a temperature of 900°C. All NaNbO₃ products were prepared from stoichiometric ratios of Nb₂O₅ and Na₂CO₃ together with the addition of a salt flux introduced at a 10:1 molar ratio of salt to NaNbO₃, that is, using the Na₂SO₄, NaF, NaCl, and NaBr salts. A solid-state synthesis was performed in the absence of a molten salt to serve as a control. The reaction products were all found to be phase pure through powder X-ray diffraction, for example, with refined lattice constants of a = 5.512(5) Å, b = 5.567(3) Å, and c = 15.516(8) Å from the Na₂SO₄ salt reaction. The products were characterized using UV-Vis diffuse reflectance spectroscopy to have a bandgap size of ~3.5 eV. The particles sizes were analyzed by scanning electron microscopy (SEM) and found to be dependent upon the flux type used, from ~<1 μm to >10 μm in length, with overall surface areas that could be varied from 0.66 m²/g (for NaF) to 1.55 m²/g (for NaBr). Cubic-shaped particle morphologies were observed for the metal halide salts with the set of exposed (100)/(010)/(001) crystal facets, while a truncated octahedral morphology formed in the sodium sulfate salt reaction with predominantly the set of (110)/(101)/(011) crystal facets. The products were found to be photocatalytically active for hydrogen production under UV-Vis irradiation, with the aid of a 1 wt% Pt surface cocatalyst. The platinized NaNbO₃ particles were suspended in an aqueous 20% methanol solution and irradiated by UV-Vis light (λ > 230 nm). After 6 hours of irradiation, the average total hydrogen production varied with the particle morphologies and sizes, with 753 µmol for Na₂SO₄, 334 µmol for NaF, 290 µmol for NaCl, 81 µmol for NaBr, and 249 µmol for the solid-state synthesized NaNbO₃. These trends show a clear relationship to particle sizes, with smaller particles showing higher photocatalytic activity in the order of NaF > NaCl > NaBr. Furthermore, the particle morphologies obtained from the Na₂SO₄ flux showed even higher photocatalytic activity, though having a relatively similar overall surface area, owing to the higher activity of the (110) crystal facets. The apparent quantum yield (100 mW/cm², λ = 230 to 350 nm, pH = 7) was measured to be 3.7% for NaNbO₃ prepared using the NaF flux, but this was doubled to 6.8% when prepared using the Na₂SO₄ flux. Thus, these results demonstrate the powerful utility of flux synthetic techniques to control particle sizes and to expose higher-activity crystal facets to boost their photocatalytic activities for molecular hydrogen production.     

Two‐step Synthesis of Platelike Potassium Sodium Niobate Template Particles by Hydrothermal Method

Two‐step hydrothermal synthesis of platelike potassium sodium niobate (K, Na)NbO₃ (KNN) template particles was investigated. Platelike K₄Na₄Nb₆O₁₉·9H₂O (KNN‐hydrate) particles were synthesized in 4 mol/L aqueous alkali at 150°C by the sodium dodecyl benzene sulfonate (SDBS) surfactant‐assisted hydrothermal method, which were used as crystal nucleus in the second step of hydrothermal synthesis. The two‐step synthesized KNN‐hydrate particles with 0.6 μm thickness and 7 μm width were prepared at 80°C after 10 h of the second step. After calcination of the KNN‐hydrate particle at 600°C, platelike KNN particles were obtained, which were used as templates for textured ceramics. Particles obtained by the two‐step synthesis showed regular morphology and uniform distribution, with a marked improvement in grain size.     

0.74NaNbO3–0.26Sr(Mg1/3Nb2/3)O3 lead-free dielectric ceramics with high energy storage properties

Sodium niobate (NaNbO₃) ceramics are commonly investigated for use as energy storage ceramics because of their excellent properties. NaNbO₃ ceramics are modified mainly by doping with a Bi-based composite perovskite, that is, by the nonequivalent doping of Bi³⁺ at the A site of the NaNbO₃ ceramic. In addition, the high volatility of Bi at high temperatures increases the defects in the ceramics. This paper provides a new idea of doping modification of sodium NaNbO₃-based energy storage ceramics. Here, (1?x)NaNbO₃–xSr(Mg_1/3Nb_2/3)O₃ (x = 0.17, 0.20, 0.23, 0.26) ceramics were prepared by doping NaNbO₃ with an Sr-based composite perovskite. Compared with Bi-based composite perovskite, Sr-based composite perovskite doping of NaNbO₃ ceramics can also obtained good energy storage properties: a total energy storage density of 4.28 J/cm³ and an energy storage efficiency of 89.3%. In addition, the ceramics exhibited good frequency stability (2–200 Hz) and a high charge/discharge rate (1.06 μs).     

Ternary doping of Na+, Bi3+ and La3+ to improve piezoelectric constant and electrical resistivity simultaneously in calcium bismuth niobate ceramics

High Curie-temperature layer-structured calcium bismuth niobate (CaBi₂Nb₂O₉) piezoelectric ceramics are promising for important application in high-temperature vibration sensors. However, such application is currently limited due to not only poor high-temperature piezoelectric constant (d₃₃), which is attributable to spontaneous polarization along a-b plane and high coercive fields, but also inferior high-temperature electrical resistivity, which results from volatilization of Bi₂O₃ during the sintering process that increases defect concentration of oxygen vacancies. Herein, we report a Na⁺, Bi³⁺ and La³⁺ ternary-doping-strategy to obtain Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics, which exhibited higher piezoelectric constant and larger electrical resistivity as accompanied by a better thermal stability at high-temperatures. The piezoelectric constant was enhanced from 8.8 pC/N in pristine CaBi₂Nb₂O₉ to 13.4 pC/N in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics, which is ascribed to the presence of pseudo-tetragonal structural distortion after La³⁺ doping. In addition, the electrical resistivity at 600 °C was increased by more than one-order of magnitude from 3.7 × 10⁴ Ω cm in pristine CaBi₂Nb₂O₉ to 1.4 × 10⁶ Ω cm in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics. Such significant improvement in electrical resistivity results from the reduction in oxygen vacancies due to ternary doping of Na⁺, Bi³⁺ and La³⁺ and stronger binding interaction between La³⁺ dopants and O^2? in (Bi₂O₂)²⁺ layers in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics. This work demonstrates an important way of employing chemical doping to improve piezoelectric constant and electrical resistivity simultaneously at high-temperatures to tune structural distortion in bismuth-layered structural CaBi₂Nb₂O₉ ceramics.     

Role of Alumina in the Preparation of Platelike NaNbO3 Powder by Molten Salt Synthesis and Proposal of New Preparation Method

The preparation of platelike NaNbO₃ grains via single‐step molten salt synthesis using Bi₂O₃, Na₂CO₃, Nb₂O₅, and NaCl as reactants was examined. When a new alumina crucible was used, platelike NaNbO₃ grains were obtained, but a repeatedly used alumina crucible resulted in irregularly shaped NaNbO₃ grains. When a platinum crucible was used, even NaNbO₃ could not be obtained. Addition of alumina substrates and alumina granules to the reaction mixture in the platinum crucible resulted in the formation of platelike NaNbO₃ grains and second‐phase grains. The second‐phase grains, which were composed of Al₂O₃, Bi₂O₃, Na₂O, and Nb₂O₅ and had a pyrochlore structure, could be removed by sieving. The second phase acted as a scavenger for Bi₂O₃ and hence, the possibility of using another scavenger was attempted. The new scavenger was a mixture of Na₂CO₃ and Nb₂O₅, and using them, platelike NaNbO₃ grains were successfully obtained with NaNb₅Bi₂O₁₆ as a byproduct, which could then be removed by sieving.     

Hydrothermal synthesis of potassium–sodium niobate powders

Potassium–sodium niobates (K_xNa_1−xNbO₃, 0 < x < 1, KNN) were hydrothermally synthesized under varying alkaline ratios (K⁺/Na⁺), total hydroxide concentration, reaction temperature, and time. Compositional surveys were developed by using Rietveld analyses derived quantitative volume fractions. The data demonstrated that phase pure KNN synthesis can be achieved by reacting the niobium source with the hydroxide solution having 6 M total hydroxide concentration, cation ratio (K⁺/Na⁺) of above 6 at temperatures ≥200°C for 24 h. Dissolution–precipitation events through intermediate products including hexaniobates were postulated as a plausible formation mechanism. It was shown also that the single-phase KNN approaching the morphotropic phase boundary (MPB) could be obtained by further incorporation of sodium ions into the crystal via post-annealing at 800°C/2 h, following the hydrothermal synthesis.     

Effects of excess K2O and processing conditions on the preparation of dense K(Ta1−XNbX)O3 ceramics

Dense ceramics of potassium tantalate niobates (KTa_1−XNb_XO₃: X = 0.4) with a perovskite-type structure were prepared using powders including excess K₂O and surrounding powder, and by applying vibration to the surrounding powder prior to firing. Excess K₂O was inevitable for increasing density, and the use of surrounding powder reduced density fluctuation to a considerable extent. Furthermore, the powder including 8 mol% excess K₂O fired at 1150 °C led to dense ceramics with relative density higher than 90%. The chemical composition of the resulting dense ceramics prepared here was confirmed to be that of a desired perovskite, KTa_0.6Nb_0.4O₃. The KTa_1−XNb_XO₃ ceramics obtained here showed almost the same dielectric properties as those prepared by delicate, costly, and time-consuming methods.     

Corrosion Behaviour of Niobium in Sodium Hydroxide Solutions

The electrochemical behaviour of niobium was investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that Nb is spontaneously active in 10, 15 and 30 wt % NaOH at 25, 50 and 75 °C. The anodic polarization curves in all cases show a dissolution/passivation peak followed by a current plateau, corresponding to Nb₂O₅ formation. The spontaneous active corrosion of Nb leads to the formation of soluble niobates that precipitate to sodium niobates. The evaluation of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicates that the corrosion rates of niobium increase with increasing NaOH concentration and temperature.     

A study of the mechanochemical synthesis of NaNbO3

We have studied the mechanochemical synthesis of NaNbO₃, prepared from a powder mixture of Na₂CO₃ and Nb₂O₅. The formation of NaNbO₃ during milling was followed using thermal analysis and X-ray diffraction. According to the thermogravimetric analysis, after 20 and 40 h of milling there was still some residual carbonate, while the X-ray diffraction shows NaNbO₃ as the major crystalline phase present in the mixture. Based on the quantitative XRD phase analysis, the residual reactants were amorphous and as such undetectable with the X-ray diffraction. Furthermore, an X-ray line-broadening analysis was used to determine the NaNbO₃ crystallite size and the microstrain. A decrease in the NaNbO₃ crystallite size coupled with an increase in the amount of microstrains was found from 10 to 40 h of mechanochemical treatment. Finally, the TEM analysis confirmed the NaNbO₃ crystallite size determined by the X-ray line-broadening analysis.