Phase formation of V2O5·xNb2O5 compounds via gels and freeze-dried precursors

An X-ray powder diffraction study of the phase formation in the system V₂O₅/Nb₂O₅ is performed. Freeze-dried ammonium vanadate and ammonium oxalato niobate, alkoxide-derived xerogels and a mixture of active oxides are used as precursors to compare the resulting phase composition. Thermal decomposition of the freeze-dried precursor is monitored with DTA/TG and mass spectrometry. In the quasi-binary system V₂O₅-Nb₂O₅ metastable VNbO₅, V₄Nb₁₈O₅₅, VNb₉O₂₅ and solid solutions of V₂O₅ in TT-Nb₂O₅ as also thermodynamically stable VNb₉O₂₅ exist. The thermal decomposition of freeze-dried vanadate-oxalatoniobate solution allows the synthesis of all these phases in a relative simple manner. Structural relationships between an intermediate phase and the product, or, in the case of solid-state reactions, between one of the starting oxide and the product, favour the desired reaction. Therefore, the structure of a former phase influences or directs the structure of the product similar to a topotactic reaction.     

Molten salt synthesis of anisometric particles in the SrO-Nb2O5-BaO system

Acicular particles of KSr₂Nb₅O₁₅, Sr_0.5Ba_0.5Nb₂O₆, and SrNb₂O₆ were synthesized in the SrO-BaO-Nb₂O₅ system, using KCl or SrCl₂·6H₂O salts. In the SrCl₂·6H₂O + Nb₂O₅ system, acicular SrNb₂O₆ particles were formed by a solid state reaction between the salt and Nb₂O₅. Large, irregularly shaped (Sr-rich) Sr₂Nb₂O₇ particles formed with increasing reaction temperature and time. Small but finite solubility of SrO in the KCl + SrNb₂O₆ system favored the formation of acicular KSr₂Nb₅O₁₅ and blade-like Sr₂Nb₂O₇ particles (at higher temperatures). Uniformly sized, acicular KSr₂Nb₅O₁₅ particles were easier to reproduce compared to the formation of acicular Sr_0.5Ba_0.5Nb₂O₆ and SrNb₂O₆ particles.     

Large-scale fabrication of H2(H2O)Nb2O6 and Nb2O5 hollow microspheres

Hollow micro-sized H₂(H₂O)Nb₂O₆ spheres constructed by nanocrystallites have been successfully synthesized via a bubble-template assisted hydrothermal process. In the reaction process, H₂O₂ acts as a bubble generator and plays a key role in the formation of the hollow structure. An in situ bubble-template mechanism has been proposed for the possible formation of the hollow structure. The spherelike assemblies of these H₂(H₂O)Nb₂O₆ nanoparticles have been transformed into their corresponding pseudohexagonal phase Nb₂O₅ through a moderate annealing dehydration process without destroying the hierarchical structure. Optical properties of the as-prepared hollow spheres were investigated. It is exciting that the absorption edge of the hollow Nb₂O₅ microspheres shifts about 18 nm to the violet compared with bulk powders in the UV/vis spectra, indicating its superior optical properties.     

New solid acids in the triple-layer Dion-Jacobson layered perovskite family

Dion-Jacobson type layered perovskites such as A′Ca₂Nb₃O₁₀ (A′ = K, Rb, H) have continued to be of great interest due to their compositional variability, rich interlayer chemistry, and wide range of physical properties. In this study, we investigated the range and effects of substitutional doping of Ta⁵⁺ for Nb⁵⁺ and of Sr²⁺ for Ca²⁺ in A′Ca₂Nb₃O₁₀. We have prepared and characterized three new solid solutions: KCa₂Nb_3−xTa_xO₁₀, RbCa₂Nb_3−xTa_xO₁₀, and RbCa_2−xSr_xNb₃O₁₀. These materials all readily undergo proton exchange to form two new series of hydrated solid acid phases, which in most cases can be dehydrated to form stable HCa₂Nb_3−xTa_xO₁₀ and HCa_2−xSr_xNb₃O₁₀ compounds. Intercalation studies with n-hexylamine and pyridine were carried out to gauge the relative Brønsted acidities across the HCa₂Nb_3−xTa_xO₁₀ series, and we determined that materials with the highest tantalum contents are weaker acids than the parent compound HCa₂Nb₃O₁₀. Preliminary intercalation studies with pyridine for the HCa_2−xSr_xNb₃O₁₀·yH₂O solid acids, however, showed no significant difference in acidity with varying strontium content.     

Synthesis of shape-controlled Nb3O7F/NbB2 heterostructure: A new idea to synthesize binary hybrid materials by incomplete reaction

Niobium oxide fluoride/niobium diboride (Nb₃O₇F/NbB₂) heterostructures with urchin-like and nanowall-like morphologies were synthesized by a facile hydrothermal approach. The high-density one-dimensional Nb₃O₇F nanoneedle arrays and two-dimensional Nb₃O₇F nanosheets stand on the surface of NbB₂ cores. Here a new idea is proposed to synthesize binary heterostructure by in situ “incomplete reaction”. Ultraviolet-visible spectra showed that such heterostructure has a wide absorption peak at around 270 nm and the absorption edge of the products synthesized at higher temperature shifts to longer wavelength because of stronger nanometric effect.     

Towards modeling gadolinium-lead-borate glasses

Infrared spectra of gadolinium-lead-borate glasses of the xGd₂O₃·(100 − x)[3B₂O₃·PbO] system, where x = 0, 5, 10, 15, 25, 35 and 50 mol.%, have been recorded to explore the role of content of gadolinium ions behaving as glass modifier.The FTIR spectroscopy data for the xGd₂O₃·(1 − x)[3B₂O₃·PbO] glasses show the structural role of lead ions as a network-formers and of the gadolinium ions network modifiers. Adding of the rare earth ion up to 35 mol.% into the glass matrix, the IR bands characteristic to the studied glasses become sharper and more pronounced.Structural changes, as recognized by analyzing band shapes of IR spectra, revealed that Gd₂O₃ causes a change from the continuous borate network to the continuous lead-borate network interconnected through Pb-O-B and B-O-B bridges and the transformation of some tetrahedral [BO₄] units into trigonal [BO₃] units. Then, gadolinium ions have affinity towards [BO₃] structural units which contain non-bridging oxygens necessary for the charge compensation because the more electronegative [BO₃] structural units were implied in the formation of B-O-Gd bonds and the transformation of glass network into a glass ceramic.We propose a possible structural model of building blocks for the formation of continuous random 3B₂O₃·PbO network glass used by density functional theory (DFT) calculations.DFT calculations show that lead atoms occupy three different sites in the proposed model. The first is coordinated with six oxygen atoms forming distorted octahedral geometries. The second lead atom has an octahedral oxygen environment and the five longer Pb-O bonds are considered as participating in the metal coordination scheme. The third lead atom has ionic character. In agreement with the results offered by the experimental FTIR data, the theoretical IR data confirm that our proposed structure is highly possible.     

The chemical stability and conductivity of BaCe0.9−xYxNb0.1O3−σ proton-conductive electrolyte for SOFC

BaCe_0.8Y_0.2O_3−δ and BaCe_0.9−xY_xNb_0.1O_3−δ (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.35) were prepared by a solid-state reactions. It was found that the BaCe_0.8Y_0.2O_3−δ samples decomposed into CeO₂ and BaCO₃ after being exposed in the atmosphere (3% CO₂ + 3% H₂O + 94% N₂) at 700 °C for 10 h. However, samples containing Nb remains unchanged in the same conditions, demonstrating a better stability in the presence of CO₂ and H₂O. The conductivity of BaCe_0.9−xY_xNb_0.1O_3−δ increased with the increase of Y content (x ≤ 0.30), and the highest value was observed at x = 0.30 where a significant decrease in conductivity took place at x = 0.35. The conductivity of BaCe_0.6Y_0.3Nb_0.1O_3−δ reaches 0.01 S/cm in humid hydrogen at 700 °C, slight lower than BaCe_0.8Y_0.2O_3−δ, 0.012 S/cm in the same conditions. Fuel cell with BaCe_0.6Y_0.3Nb_0.1O_3−δ as-prepared was successfully prepared and humidified hydrogen was supplied as fuels in evaluating the fuel cell performance. The open circuit voltage, peak power density and interfacial resistance at 700 °C were 1.02 V, 345 mW/cm² and 0.27 Ω cm², respectively.     

Composition dependent phase connectivity, dielectric and magnetoelectric properties of magnetoelectric composites with Pb(Mg1/3Nb2/3)0.67Ti0.33O3 as piezoelectric phase

Ferrite (Ni_0.6Co_0.4Fe₂O₄) phase, ferroelectric (Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃) phase and magnetoelectric composites of (x)Ni_0.6Co_0.4Fe₂O₄ + (1 − x)Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ with x = 0.15, 0.30 and 0.45 were prepared using solid-state reaction technique. Presence of Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ was confirmed using X-ray diffraction technique. The scanning electron microscopic images were used to study the microstructure of the composites. Connectivity scheme present in the magnetoelectric (ME) composites are discussed from the microscopic images. Variation of dielectric constant and dielectric loss with temperature for all the composites was studied. Here we report the effect of Ni_0.6Co_0.4Fe₂O₄ mole fraction on connectivity schemes between Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composite. The variation of magnetoelectric voltage coefficient with dc magnetic field shows peak behaviour. The maximum value of magnetoelectric voltage coefficient of 9.47 mV/cm Oe was obtained for 0.15Ni_0.6Co_0.4Fe₂O₄ + 0.85Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composites. Finally we have co-related the effect of Ni_0.6Co_0.4Fe₂O₄ content and dielectric properties on magnetoelectric voltage coefficient.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

Low temperature preparation of nanocrystalline Sr0.5Ba0.5Nb2O6 powders using an aqueous organic gel route

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) ceramic powders have been synthesized by an aqueous organic gel route. Homogeneous Sr-Ba-Nb precursor gels are prepared with Ba-EDTA, Sr-EDTA, and Nb-citrate complex as source of Sr, Ba, and Nb, respectively. Citric acid and ethylenediaminetetraacetic acid (EDTA) were used as the chelating agents. The structural variation of the SBN powder with annealing temperature was studied by TG-DTA, FT-IR and XRD. The precursor gel on calcination at 800 °C for 2 h produces a pure tungsten bronze SBN phase and the corresponding average particle size is 30-50 nm. The influences of the pH and the molar ratio of citric acid:Nb cation on the formation of homogeneous Sr-Ba-Nb precursor gels were also studied. The results show that a homogeneous Sr-Ba-Nb precursor gel with no precipitate is formed at pH 8 and the optimum molar ratio of citric acid and the metal cations is 3:1.     

Structure analysis on the Ba3Mg(Ta1−xNbx)2O9 ceramics: Coexistence of order and disorder

The Ba₃ZnTa₂O₉ (BZT) and Ba₃MgTa₂O₉ (BMT) ceramics, a family of A₃B²⁺B⁵⁺₂O₉ complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb⁵⁺ at the Ta⁵⁺ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb⁵⁺ content have significant effect on the ordering of B cations in the Ba₃Mg(Ta_1−xNb_x)₂O₉ solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb⁵⁺ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B²⁺ and B⁵⁺ cations.     

Preparation of magnetite core–shell nanoparticles of Fe3O4 and carbon with aryl sulfonyl acetic acid

High quality Fe₃O₄/carbon core–shells and shell–core nanoparticles have been successfully synthesized by depositing an epitaxial growth of Fe₃O₄ or carbon shell onto carbon or Fe₃O₄ nanocore. By employing the agents such as aryl sulfonyl acetic acid and glucose, Fe₃O₄ and carbon in a nanoscale was prepared from iron aryl sulfonyl acetate and then by the solvothermal reaction of glucose in a reverse microemulsion. The advantages of present approach rely not only on its simplicity, rapidity, and efficiency of the procedure, but also the formation of the controlled core–shell structures as well. It is highly suitable for further applications. Different core–shell structure controls could be attained by careful adjustment of the procedure sequences of decarboxylation and solvothermal reaction. The magnetic studies show that Fe₃O₄/carbon core–shell and shell–core nanoparticles found to be superparamagnetic. The characteristic differences in the core–shell structures would lead to the change of magnetization behaviors of Fe₃O₄ nanoparticles.     

Preparation of KNbO3 by hydrothermal reaction

Perovskite-type KNbO₃ powder was prepared by hydrothermal reaction using Nb₂O₅ in KOH solution. A single phase of KNbO₃ was obtained when the molar ratio of KOH/Nb₂O₅ was above 20 and the reaction temperature was above 160 °C. Three types of KNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, depending on the reaction temperature and the ratio of KOH/Nb₂O₅. The molar ratios of K/Nb in the cubic and tetragonal phases were 0.91 and 0.94, respectively and that of the orthorhombic one was 0.98, and the mass loss was observed in the TG curves of tetragonal and cubic phases. The tetragonal and cubic phases were stabilized by OH⁻ and adsorbed water.     

The interaction of hydrogen with oxidic promoters of hydrogen storage in magnesium hydride

The present work is devoted to monitor the reactivity with hydrogen of various oxide systems (Nb₂O₅, WO₃ and a mixed Mg/Nb/O oxide) in the aim of understanding the role of oxides as promoters for hydrogen storage in MgH₂. The reactivity of the oxides has been tested using either molecular hydrogen or “nascent” hydrogen produced by reaction of zinc with hydrochloric acid. Thermal desorption-mass spectrometry experiments indicate that hydrogen adsorbed in Nb₂O₅ and Mg/Nb/O mixed oxides is, in part, reversibly released and, for a second fraction, released as water. By contrast, water is the only product desorbed by WO₃ after contact with hydrogen. This could explain the higher performances of Nb₂O₅ as kinetic promoter of hydrogen storage in comparison with WO₃.     

The influence of ultrasound on the formation of TiO2 nanotube arrays

Electrochemically anodized TiO₂ nanotube (NT) arrays on Ti foils have been in great interests recently attributing to the wide applications. The net growth rate of TiO₂ NT array is determined by electrochemical etching of Ti foil and chemical dissolution of the formed TiO₂ NTs. While the TiO₂ NT growth becomes a diffusion controlled process after certain growth time. Here, we report the influence of ultrasound on the growth process of TiO₂ NT arrays. The results indicate that ultrasound can significantly alter the relative rates of these reactions and adjust the geometries of the TiO₂ NT arrays.     

Crystal structures of KBa2Nb5O15 and LaK2Nb5O15 with the TTB-type structure

Single crystals of two niobates, KBa₂Nb₅O₁₅ and LaK₂Nb₅O₁₅, were synthesized by high-temperature reaction and the crystal structures were determined by single crystal X-ray diffraction data. Although the space groups for these compounds were different (the non-centrosymmetrical space group P4bm (#100) for KBa₂Nb₅O₁₅ and the centrosymmetrical one P4/mbm (#127) for LaK₂Nb₅O₁₅), both compounds had the same tetragonal tungsten bronze-type (hereafter TTB-type) structure. The lattice parameters and R-factors of KBa₂Nb₅O₁₅ (LaK₂Nb₅O₁₅) were a = 12.533(2) (12.563(2)) and c = 4.0074(9) (3.9179(9)) Å, and R₁ = 0.040 (0.047) and wR₂=0.131 (0.120), respectively. From the crystal structural analysis, it was clarified that distribution of two large cations was different from each other in the way that K and Ba atoms in KBa₂Nb₅O₁₅ were distributed statistically at two crystallographic sites and K and La atoms in LaK₂Nb₅O₁₅ were ordered.     

Lead magnesium niobate-lead titanate fibres by a modified sol-gel method

Lead magnesium niobate-lead titanate (PMN-PT) ceramic fibres with the nominal composition of 0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ have been fabricated by a modified sol-gel method. Due to the difficulty of dissolving the magnesium component, the mixed oxide method was used together with the traditional sol-gel method. To obtain crack-free fibres, pyrolysis was carried out at a very slow heating rate under specific atmosphere to control the organic burnout. The thermal and microstructural properties were investigated using thermogravimetric analysis, scanning electron microscopy and X-ray diffraction. The optimum sintering temperature is 1200 °C and yields a fibre with a final diameter of around 100 μm. A single PMN-PT fibre has been poled and its electrical properties were measured. The properties of the fibre are found to be better than that of a ceramic disc.     

Effect of MnO2 addition on the piezoelectric properties in Pb(Mg1/3Nb2/3)O3 relaxor ferroelectrics

The effects of MnO₂ addition on the piezoelectric properties in Pb(Mg_1/3Nb_2/3)O₃ relaxor ferroelectrics were studied in the ferroelectricity-dominated temperature range from −40 to 30°C. Dielectric, piezoelectric, and electrostrictive properties were examined to clarify the effect of MnO₂ addition. As an added amount of MnO₂ increases, the dielectric constant decreases and the mechanical quality factor increases in Pb(Mg_1/3Nb_2/3)O₃. From the experimental results, it has been found that Mn behaves as a ferroelectric domain pinning element.     

Dielectric characteristics of bismuth-modified lead magnesium niobate ceramics

Aliovalent Bi was substituted for Pb in Pb(Mg_1/3Nb_2/3)O₃ with required alteration in the Mg/Nb ratio. Resultant changes in the perovskite developments, lattice parameters as well as dielectric characteristics were investigated. Powders were prepared via a two-step B-site precursor route to enhance the perovskite formation. The perovskite structure persisted up to the range of 30 mol% Bi(Mg_2/3Nb_1/3)O₃ substitution. Values of the maximum dielectric constant decreased drastically, while the dielectric maximum temperatures changed only moderately. Meanwhile, the diffuseness exponent values decreased continuously with the Bi modification.