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.     

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⁻].     

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.     

Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders

NaNbO₃, (K,Na)NbO₃ and KNbO₃ powders were synthesized using (1− y) NaOH–y KOH solutions ([OH⁻] =7.5–15 M) with y=0, 0.78, and 1 at 200 °C by the hydrothermal method, respectively. Their compositions, structures, and morphologies were analysed. Both of the synthesized NaNbO₃ and KNbO₃ powders had sub-micron- or micron-sized grains. The [OH⁻] drastically influenced the size and morphology of the KNbO₃ particles but did not influence those of the NaNbO₃ particles. In contrast, the morphology of the (K,Na)NbO₃ particles, which were aggregates of nano-grains, was influenced by the hydrothermal-treatment time rather than [OH⁻]. Moreover, their composition and phase were influenced by both annealing and the hydrothermal-treatment time, and their formation mechanism was discussed by comparison with those of KNbO₃ and NaNbO₃ particles. The present synthetic strategy enables tailoring the compositions, morphologies, and structures of the niobate products to different applications by controlling the process parameters.     

Sodium niobates and protonic niobates nanowires obtained from hydrothermal synthesis: Electrochemical behavior in aqueous electrolyte

Niobium-based oxides can be used in several applications due to a diverse set of properties. In this work, Na₂Nb₂O₆.H₂O sodium niobate nanowires were obtained by hydrothermal synthesis at low temperature. Dehydrated sodium niobate (Na₂Nb₂O₆) and sodium niobate with perovskite structure (NaNbO₃) were obtained by submitting Na₂Nb₂O₆.H₂O to heat treatment (350 °C and 500 °C, respectively). To obtain protonic niobates, sodium niobates were immersed in nitric acid in order to promote ion exchange reactions. From this procedure, protonic niobates (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ were obtained. The sample NaNbO₃ did not undergo any transformation. Cyclic voltammetry tests carried out in a neutral aqueous solution 1 M Na₂SO₄ showed a wide potential window for both niobates (sodium and protonic). However, the protonic niobate samples (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ presented much higher current density values than the sodium niobates. This result can be related to a structural rearrangement that allowed a significant increase in the intercalation of sodium Na ⁺ ions from the electrolyte into the structure of these protonic niobates, when polarized. Therefore, in this work, it was demonstrated that it is possible to obtain protonic niobates from sodium niobates, as well as, it was verified the distinct electrochemical behavior between these materials.     

Preparation and electrical properties of NaNbO3 ceramics synthesized by topochemical microcrystal conversion

NaNbO₃ (NN) ceramics were successfully synthesized from bismuth layer-structured Bi_2.5Na_3.5Nb₅O₁₈ (BNN) particles by a topochemical microcrystal conversion method. Plate-like BNN particles were first synthesized by molten salt synthesis (MSS) technique. After topochemical reaction with complementary reactant sodium carbonate (Na₂CO₃) in sodium chloride (NaCl) flux, the layer structure of BNN particles were transformed to NN perovskite structure. The crystal structure and microstructure of the synthesized particles were examined through XRD and SEM analysis, respectively. Dielectric, complex impedance and conduction behaviors of the sintered NN ceramics were investigated in the frequency range of 100 Hz–1 MHz at different temperatures (50–550 °C).     

Synthesis and characterization of Nb2O5@C core-shell nanorods and Nb2O5 nanorods by reacting Nb(OEt)5 via RAPET (reaction under autogenic pressure at elevated temperatures) technique

The reaction of pentaethoxy niobate, Nb(OEt)₅, at elevated temperature (800 °C) under autogenic pressure provides a chemical route to niobium oxide nanorods coated with amorphous carbon. This synthetic approach yielded nanocrystalline particles of Nb₂O₅@C. As prepared Nb₂O₅@C core-shell nanorods is annealed under air at 500 °C for 3 h (removing the carbon coating) results in neat Nb₂O₅ nanorods. According to the TEM measurements, the Nb₂O₅ crystals exhibit particle sizes between 25 nm and 100 nm, and the Nb₂O₅ crystals display rod-like shapes without any indication of an amorphous character. The optical band gap of the Nb₂O₅ nanorods was determined by diffuse reflectance spectroscopy (DRS) and was found to be 3.8 eV.     

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.     

Formation mechanism and growth of MNbO3, M=K,Na by in situ X‐ray diffraction

Hydrothermal synthesis is a well‐established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead‐free piezoelectric K_0.5Na_0.5NbO₃. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end‐members, KNbO₃ and NaNbO₃, are nucleating and growing. Here, we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO₃ and NaNbO₃ by in situ synchrotron powder X‐ray diffraction. We show that tetragonal KNbO₃ crystallites form from dissolved T‐Nb₂O₅ at 250°C‐300°C and 250 bar while orthorhombic NaNbO₃ forms via several crystalline intermediate phases at 225°C‐325°C and 250 bar. The crystallite size of KNbO₃ is decreasing while the crystallite size of NaNbO₃ is increasing with increasing temperature, demonstrating that the presence of intermediate phases is highly important for the nucleation and growth of the final product. The different crystallization schemes explain the challenge in obtaining stoichiometric K_0.5Na_0.5NbO₃ by hydrothermal synthesis.     

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.     

Crystallization Atmosphere and Substrate Effects on the Phase and Texture of Chemical Solution Deposited Strontium Niobate Thin Films

Strontium niobate (Sr:Nb  =  1:1) thin films were prepared via chemical solution deposition on (001)‐oriented SrTiO₃, (001)_p‐oriented LaAlO₃, (0001)‐oriented sapphire, and polycrystalline alumina substrates. Crystallization in oxygen at 1000°C yielded Sr₂Nb₂O₇ films on all substrates with strong (010) orientation. Films on LaAlO₃ and SrTiO₃ single‐crystal substrates possessed a small amount of preferred in‐plane orientation, whereas films prepared on sapphire and polycrystalline alumina substrates were fiber textured. Films crystallized at 900°C in a low oxygen atmosphere (~10 ^{? 21} atm pO₂) formed a randomly oriented polycrystalline perovskite, SrNbO_3?δ on all substrates. A similar set of films crystallized at 900°C at a slightly higher oxygen partial pressure (~10^?15 atm pO₂) was comprised of Sr₂Nb₂O₇ and SrNbO_3?δ phases, exposing the dependence of phase formation on oxygen partial pressure. When subjected to a high‐temperature anneal in oxygen, the SrNbO_3?δ phase is shown to transform into Sr₂Nb₂O₇, however, Sr₂Nb₂O₇ did not significantly reverse transform into SrNbO_3?δ after annealing in low oxygen partial pressure atmospheres.     

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.     

Processing and Properties of Textured Potassium Sodium Niobate [K,Na]NbO3 Ceramic Ribbons by Alginate Gelation Method

Random and <001> textured potassium sodium niobate – [K,Na]NbO₃ (KNN) ceramics with 1 mole% CuO sintering aid were fabricated in ribbon form through a combination of novel alginate gelation process and templated grain growth methods using platelike sodium niobate ‐ NaNbO₃ (NN) template particles. The platelike NN template particles were prepared by a two‐step molten salt synthesis method. Ribbons were drawn from alginate‐based slurries without or with 10 wt% NN template particles using 50 mm long slit nozzle with a rectangular orifice of 10 mm × 1 mm. Development of crystallographic texture as a result of varying sintering time and temperature was evaluated through the calculation of the degree of orientation as measured by the Lotgering factor (?₍₀₀₁₎) and an ?₍₀₀₁₎ of 0.81 was achieved. The electrical properties of textured ribbons were evaluated with polarization and strain versus electric field measurements.     

Tailoring structure and piezoelectric properties of CaBi2Nb2O9 ceramics by W6+-Doping

Calcium bismuth niobate (CaBi₂Nb₂O₉) is a typical bismuth-layer structured piezoelectrics (BLSPs) with a high Curie temperature (T_C) of ～943 °C, but it has low piezoelectric coefficient and high-temperature resistivity which severely limits signal acquisition in the high-temperature piezoelectric vibration sensors. Ion-doping modification is regarded as an effective way to enhance electrical properties. In this work, W⁶⁺ donor-doping at Nb⁵⁺ site in the CaBi₂Nb_2-xW_xO₉ (x = 0, 0.020, 0.025, 0.030, 0.035 and 0.040) piezoelectric ceramics with T_C of 931 ± 2 °C were fabricated by the conventional solid-state reaction method. The effects of W⁶⁺-doping on crystal structure of CaBi₂Nb_2-xW_xO₉ as well as microscopic morphology and electrical properties of ceramics were investigated systematically. The tetragonality, isotropy and electrical properties of the ceramics were enhanced with the introduction of W⁶⁺ dopant. It was found that x = 0.025 was the optimal W⁶⁺-doping ratio that yielded remnant polarization of 8.0 μC/cm², electrical resistivity of 3.0 × 10⁶ Ω cm at 600 °C, piezoelectric coefficient (d₃₃) of 14.4 pC/N, and good thermal depoling property. Our work has established a feasible approach to tune the structure of CaBi₂Nb₂O₉ to improve piezoelectric properties for potential applications in high-temperature piezoelectric vibration sensors.     

Ultrahigh electric breakdown strength,excellent dielectric energy storage density,and improved electrocaloric effect in Pb-free (1-x)Ba(Zr0.15Ti0.85)O3-xNaNbO3 ceramics

In this study, NaNbO₃ (NN) was introduced into Ba(Zr_0.15Ti_0.85)O₃ (BZT) to form a solid solution with relaxor ferroelectric characteristics. The dielectric breakdown strength (BDS) of the specimen with 6 mol.% NN reached 680 kV/cm, the corresponding recoverable energy storage density (W_rec) was 5.15 J/cm³, and the energy storage efficiency (η) was 77%. The dissolution of Na ⁺ ions at the A position and Nb⁵⁺ ions at the B position of the perovskite structure reduced the concentration of oxygen vacancies in the lattice and compensated for defects. The doped ceramics exhibited lower dielectric loss and better thermal stability: the W_rec value was 2 ± 1% J/cm³ at 30–120 °C. In particular, in the 0.02NN ceramics, a ΔT of 1.81 K was achieved at 130 kV/cm, and the operating temperature zone expanded with the increase in doping concentration. The introduction of NN resulted in BZT ceramics that possess excellent energy storage performance and electrocaloric effect properties.     

Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Enhanced energy storage properties of Bi(Ni2/3Nb1/6Ta1/6)O3–NaNbO3 solid solution lead-free ceramics

Sodium niobate energy storage ceramics with good environmental performance are widely used in electric power conversion and pulse power system, large energy storage density and high efficiency, huge power density and charge and discharge faster. In this work, (1-x)NaNbO₃-xBi(Ni_2/3Nb_1/6Ta_1/6)O₃ [(1-x)NN-xBNNT] (0.12 ≤ x ≤ 0.18) ceramics system were prepared by solid state reaction method. By introducing Bi(Ni_2/3Nb_1/6Ta_1/6)O₃ (BNNT), a relaxation strategy was constructed, which significantly improved the energy storage properties of NaNbO₃ (NN) based ceramics. Finally, comparatively high recoverable energy density (W_rec) of 3.43 J/cm³ and large energy storage efficiency (η) of 83.3% were obtained in 0.86NN-0.14BNNT ceramics. Besides discharge energy density (W_d) of 0.69 J/cm³, ultra fast charge-discharge rate (t_0.9) of 55 ns, the power density (P_D) of 70.66 MW/cm³ and the current density (C_D) of 883.23 A/cm² were also observed in ceramic.     

Effect of crystallization temperature on dielectric and energy-storage properties in SrO-Na2O-Nb2O5-SiO2 glass-ceramics

The SrO-Na₂O-Nb₂O₅-SiO₂ (SNNS) glass-ceramics were prepared through the melt-quenching combined with the controlled crystallization technique. XRD results showed Sr₆Nb₁₀O₃₀, SrNb₂O₆, NaSr₂Nb₅O₁₅ with tungsten bronze structure and NaNbO₃ with the perovskite structure. With the decrease of crystallization temperature, dielectric constant firstly increased and then decreased, while breakdown strength (BDS) was increased. High BDS of the glass-ceramics is attributed to the dense and uniform microstructure at low crystallization temperature. The optimal dielectric constant of 140±7 at 900 °C and BDS of 2182±129 kV/cm at 750 °C were obtained in SNNS glass-ceramics. The theoretical energy-storage density was significantly improved up to the highest value of 15.2±1.0 J/cm³ at 800 °C, which is about 5 times than that at 950 °C. The discharged efficiency increased from 65.8% at 950 °C to 93.6% at 750 °C under the electric field of 500 kV/cm by decreasing crystallization temperature.     

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.     

Electrical Properties of a 0.95(Na0.5K0.5)NbO3–0.05CaTiO3 Thin Film Grown on a Pt/Ti/SiO2/Si Substrate

An amorphous phase was formed in a 0.95(Na_0.5K_0.5)NbO₃–0.05CaTiO₃ (NKN‐CT) film grown at 300°C, and a low‐temperature transient Ca₂Nb₂O₇ phase was formed in the film grown at 500°C. In films grown at high temperatures (≥600°C), secondary phases such as K_5.75Nb_10.85O₃₀ and K₄Ti₁₀Nb₂O₂₇ were developed without the formation of a NKN‐CT phase, probably because of Na₂O evaporation. The same secondary phases were formed in the film grown at 300°C and subsequently annealed at 850°C under an air atmosphere. However, a homogeneous NKN‐CT phase was formed in films grown at 300°C and subsequently annealed at 830°C–880°C under the K₂O and Na₂O atmospheres. Moreover, the film annealed at 830°C in particular exhibited good electric and piezoelectric properties, including a high dielectric constant of 747 with a low dissipation factor of 0.93% at 100 kHz, low leakage current density of 2.0 × 10^?7 A/cm² at 0.1 MV/cm, and high P_r and d₃₃ values of 15.4 μC/cm² and 124 pm/V at 100 kV/cm, respectively.