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.     

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⁻].     

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.     

One-pot hydrothermal synthesis and formation mechanism of SrNb2O6–Sr2Nb2O7–Sr5Nb4O15 lamellar perovskites in highly concentrated NaOH solutions

The mechanisms behind the phase formation of Sr_xNb_yO_(x+5 y/2) perovskites in hydrothermal reactions are yet not well understood. In this work, we report on the one-pot hydrothermal synthesis of SrNb₂O₆, Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅ layered perovskites in highly concentrated NaOH solutions. The mechanisms that govern the crystallization and the crystallite morphology of the synthesized perovskites are explained as a function of the NaOH concentration. The crystal structure, morphology, and optical properties of the obtained SrNb₂O₆, Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅ perovskites were analyzed using XRD, EDS, HR-TEM, UV–vis absorption, diffuse reflectance spectroscopy (DRS), and Raman spectroscopy. The element composition and resultant crystalline phases are determined by using EDX and Rietveld refinement techniques, respectively. NaOH concentration is the main parameter for the control over the phase formation. High concentration of NaOH accelerates the hydrolysis rate for the transformation from Nb₂O₅ to Nb₆O₁₉⁸⁻ and finally to NbO₆⁷⁻. These polyanions favor the formation of the different crystalline phases, as high concentration of Nb₆O₁₉⁸⁻ the preferential formed phases are SrNb₂O₆, and Sr₂Nb₂O₇, while high concentration of NbO₆⁷⁻ induced the formation of Sr₅Nb₄O₁₅ as a majority phase. The concentration of NaOH also plays a decisive role in the precipitation of SrNb₂O₆, Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅ perovskites, high NaOH concentration promotes the Sr²⁺ complexation favoring the formation of Sr²⁺ rich phases.     

Phase Separation and Crystallization in Glasses of the Na2O–K2O–Nb2O5–SiO2 System

The structure of initial glasses in the Na₂O–K₂O–Nb₂O₅–SiO₂ system with an Nb₂O₅ content ranging from 5 to 39 mol % and their structural transformations in the course of isothermal treatments at different temperatures are investigated using small-angle X-ray scattering (SAXS) and X-ray powder diffraction. It is demonstrated that, in glasses containing 15 mol % Nb₂O₅ and more, the metastable phase separation is the primary process responsible for the formation of a microinhomogeneous structure. With further heat treatment of these glasses, NaNbO₃ crystals precipitate in regions with a high Nb₂O₅ content. In this case, each region has a heterogeneous structure and consists of NaNbO₃ microcrystals surrounded by layers of the high-silica matrix. These layers hinder transfer processes and, consequently, recrystallization, which ensures the stability of the heterogeneous structure and, correspondingly, constant sizes of microcrystals at temperatures up to 750–800°C. The intensity of light scattering is determined primarily by the sizes of regions formed upon phase separation. A decrease in their size with an increase in the niobium oxide content leads to a decrease in the light scattering loss and an increase in the transparency of heat-treated samples. The interference effects associated with the heterogeneous structure of high-niobate phase regions also favor an increase in the transparency of the prepared materials.     

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.     

A modified two-Stage mixed oxide synthetic route to lead magnesium niobate and lead iron niobate

A modified mixed oxide synthetic route has been developed for the synthesis of lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃; PMN] and lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃; PFN] powders. The formation of perovskite and pyrochlore phases in the calcined PMN and PFN powders has been investigated as a function of calcination temperature and time by XRD and DTA techniques. The particle size distribution of calcined powders was determined by laser diffraction, with the morphology, phase composition and crystal structure determined via SEM, TEM and EDX techniques. In both cases it has been found that cubic pyrochlore phases in the PbO–Nb₂O₅ system tend to form, as well as the perovskite phase. However, pyrochlore-free PMN and PFN powders were successfully obtained for a calcination temperature of 800°C for 4 and 3 h, respectively, without the introduction of excess PbO and/or MgO.     

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.     

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.     

Modulating the energy storage performance of NaNbO3-based lead-free ceramics for pulsed power capacitors

Nb-containing antiferroelectric materials have recently attracted great research interest as energy storage materials for pulsed power capacitors due to their extraordinary energy storage performances. In this case, the optimization of the energy storage performance is obtained by a compositional modulation of NaNbO₃-Bi(Zn_2/3Nb_1/3)O₃ bulk ceramics. An optimal energy performance can be obtained with a composition of 0.85NaNbO₃-0.15Bi(Zn_2/3Nb_1/3)O₃, which is accompanied by a stable charge energy density in temperatures up to 150 °C owing to its relaxor characteristics and excellent cycling stability after 10⁵ cycles. This work further broadens the scope of NaNbO₃-based ceramic applications in the area of pulsed power sources.     

Single-crystalline nanoporous Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> nanotubes

Single-crystalline nanoporous Nb₂O₅ nanotubes were fabricated by a two-step solution route, the growth of uniform single-crystalline Nb₂O₅ nanorods and the following ion-assisted selective dissolution along the [001] direction. Nb₂O₅ tubular structure was created by preferentially etching (001) crystallographic planes, which has a nearly homogeneous diameter and length. Dense nanopores with the diameters of several nanometers were created on the shell of Nb₂O₅ tubular structures, which can also retain the crystallographic orientation of Nb₂O₅ precursor nanorods. The present chemical etching strategy is versatile and can be extended to different-sized nanorod precursors. Furthermore, these as-obtained nanorod precursors and nanotube products can also be used as template for the fabrication of 1 D nanostructured niobates, such as LiNbO₃, NaNbO₃, and KNbO₃.     

Grain boundary segregation and secondary-phase transition of (La,Nb)-codoped TiO2 ceramic

The formation and transition of secondary phases in (La, Nb)-codoped TiO₂ ceramics were investigated using samples prepared by traditional solid-state sintering from anatase TiO₂, Nb₂O₅, and La₂O₃ oxide powders. From the microstructure, crystal structure and chemical composition of these samples, as measured by SEM, XRD and EDS, the formation mechanism of the secondary phase is determined though point defect thermodynamical analysis. The results show that LaNbO₄ and LaNbTiO₆ secondary phases originate from segregation of the point defects Nb_Ti^· and La′_Ti at grain boundaries, with elastic strain energy providing the main driving force for this segregation. An increase in sintering temperature causes more of the LaNbO₄ secondary phase to transition to LaNbTiO₆. A ternary phase diagram of La₂O₃–Nb₂O₅–TiO₂ was plotted based on binary phase diagrams of La₂O₃–Nb₂O₅, Nb₂O₅–TiO₂ and La₂O₃–TiO₂, which can be used to estimate the relative content of secondary phases in samples sintered at different temperatures.     

Hydrothermal synthesis of Ba5Nb4O15 ultrafine powders

Ba₅Nb₄O₁₅ ultrafine powders were prepared by hydrothermal method using Ba(NO₃)₂ and Nb₂O₅ precursors. The synthesis characteristics were influenced by the chemical form of precursors, reaction temperature, mole ratio of Ba/Nb and pH value. The chemical form of the precursors has a strong influence on the products of Ba₅Nb₄O₁₅. There was no requisite compound in the final product when Nb₂O₅ powder was used as Nb-precursor. In addition, the reaction completeness was also dependent on the synthesis temperature. The best single phase Ba₅Nb₄O₁₅ ultrafine powders were obtained at a temperature of 230 °C. The most appreciate Ba/Nb molar ratio was 3/2. The reactions were only carried out in a solution with pH value between 12 and 14. When pH was equal or larger than 14, there was no product of Ba₅Nb₄O₁₅ powder in the reactor.     

Low-temperature synthesis of ZnNb2O6 powders via hydrothermal method

ZnNb₂O₆ powder was successfully synthesized via hydrothermal method with Nb₂O₅ and Zn(NO₃)₂·6H₂O as raw materials and cyclohexane as solvent. Phase composition, morphology, and chemical composition were determined via a combination of XRD, SEM, TEM and EDS techniques. The effects of synthesis temperature and reaction time on phase composition and particle morphology were investigated in this paper. The results showed that fine ZnNb₂O₆ powders could be obtained at a hydrothermal temperature of 190 °C or above under different reaction time.     

One‐Step Surfactant‐Free Hydrothermal Synthesis of Platelike Sodium Niobate Template Powders

A one‐step surfactant‐free hydrothermal route is developed to prepare platelike NaNbO₃ template powders. At optimal KOH concentration, pure platelike NaNbO₃ with rhombohedral structure (width and thickness of 20 and 2 μm, respectively) is obtained at 200°C for 16 h. After calcination at 600°C for 4 h, the structure of the hydrothermally synthesized NaNbO₃ changes from rhombohedral to orthorhombic, whereas the initial platelike morphology is maintained. Such characteristics in terms of phase structure, elemental composition, and morphology render our hydrothermally synthesized NaNbO₃ suitable for textured ceramic fabrications.     

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.     

Preparation of Morphology‐Controlled Plate‐Like Sodium Niobate Particles by Hydrothermal Synthesis

Sodium niobate (NaNbO₃) particles with plate‐like morphology and hexagonal unit cells were prepared by the hydrothermal method. The result of SEM showed that the hexagonal NaNbO₃ were characterized by plate‐like morphology with a diameter of 5–15 μm and a thickness of 1–2 μm. The crucial influences on the morphology and crystal phase of the NaNbO₃, such as concentration of [OH^?], surfactant, and K⁺:Na⁺ ratio, were established. By further calcination treatment, the plate‐like hexagonal NaNbO₃ particles could be completely transformed into perovskite structure without morphology change. The XRD and EBSD results indicate that the major face of the calcined particles is parallel to the crystallographic (001)_pc (pseudo cubic index) plane. Compared with the traditional high‐temperature molten salt method, this work provides a simpler way to prepare the template for fabricating textured ceramics.     

Highly Selective and Active Niobia-Supported Cobalt Catalysts for Fischer–Tropsch Synthesis

The performance of Co/Nb₂O₅ was compared to that of Co/γ-Al₂O₃ for the Fischer–Tropsch synthesis at 20 bar and over the temperature range of 220–260 °C. The C₅₊ selectivity of Nb₂O₅-supported cobalt catalysts was found to be very high, i.e. up to 90 wt% C₅₊ at 220 °C. The activity per unit weight cobalt was found to be similar for Nb₂O₅ and γ-Al₂O₃-supported catalysts at identical reaction temperature. However, due to the low porosity of crystalline Nb₂O₅, the cobalt loading was limited to 5 wt% and consequently the activity per unit weight of catalyst was lower than of Co/γ-Al₂O₃ catalysts with higher cobalt loadings. This low activity was largely compensated by increasing the reaction temperature, although the C₅₊ selectivity decreased upon increasing reaction temperature. Due to the high intrinsic C₅₊ selectivity, Nb₂O₅-supported catalysts could be operated up to ~250 °C at a target C₅₊ selectivity of 80 wt%, whereas γ-Al₂O₃-supported catalysts called for an operation temperature of ~210 °C. At this target C₅₊ selectivity, the activity per unit weight of catalyst was found to be identical for 5 wt% Co/Nb₂O₅ and 25 wt% Co/Al₂O₃, while the activity per unit weight of cobalt was a factor of four higher for the niobia-supported catalyst.     

Colloidal Sol–Gel Synthesis and Photocatalytic Activity of Nanoparticulate Nb2O5 Sols

A detailed study of a novel synthesis via colloidal sol–gel route for obtaining nanoparticulate Nb₂O₅ was performed. Parameters such as temperature and H⁺:Nb⁵⁺ and Nb⁵⁺:H₂O molar ratios were controlled in order to determine the best conditions of synthesis. Moreover, particle size distribution, zeta potential, structure by X‐ray diffraction, and the photocatalytic activity of the particulate sols were also evaluated. The obtained results indicate that the colloidal sol–gel synthesis is a good alternative for obtaining Nb₂O₅ either as stable nanoparticulate sol or as a nanosized powder. Nb₂O₅ amorphous nanoparticles with an average size of 20 nm were obtained by controlling the synthesis variables. The heat‐treatment process allowed the formation of Nb₂O₅ with orthorhombic structure that transforms at higher temperatures to monoclinic phase. The highest photocatalytic activity was observed under λ = 365 nm, the smallest UV energy used in the experimental tests.     

Synthesis of γ-Bi2O3/YSZ composite powders using a facile precipitation method

Low melting point and high ionic conductivity of γ-Bi₂O₃ make it a promising additive to decrease the sintering and operation temperatures of yttria stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. Herein, γ-Bi₂O₃/YSZ composite powders with good uniformity and precise control of morphology and phase were successfully synthesized via a low cost chemical precipitation method. Both the concentration of NaOH solution and the reactant adding sequence affect the morphology and synthesis of γ-Bi₂O₃/YSZ composite powders. When the concentration of NaOH was in the range of 1.25–1.875 M, tetrahedron γ-Bi₂O₃/YSZ powders were synthesized. While, cubic structural γ-Bi₂O₃/YSZ powders were obtained when adding Bi³⁺ and YSZ suspension into 1.5 M NaOH solution. The addition of YSZ facilitates the fabrication of γ-Bi₂O₃ and widens its process window to a higher NaOH concentration. Thus synthesized γ-Bi₂O₃/YSZ composite powders effectively decrease the sintering temperature of YSZ to 1050°C due to the uniform distribution of γ-Bi₂O₃ inside YSZ powders. This work provides a facile method to fabricate γ-Bi₂O₃/YSZ composite powder with controlled morphology and phase, which will promote the mass production of low cost YSZ-based electrolyte for SOFC applications.