Papers by Keyword: Niobate

Abstract: The chlorine-doped complex oxide Ba₂CaNbO_5.475Cl_0.05 based on barium calcium niobate was synthesized using the solid state method. It was found that the introduction of chloride ions leads to the increase of the cell volume. Structure and electrical properties have been investigated. Electrical conductivities were measured by varying the temperature in dry (pH₂O=3.5·10^-5 atm) and wet (pH₂O=2·10^-2 atm) air. The composition Ba₂CaNbO_5.475Cl_0.05 is capable to dissociative dissolution of water vapor and can exhibit proton transport. Chlorine doping increases the conductivity of matrix compound Ba₂CaNbO_5.5, the difference between un-and chlorine-doped samples is up to one order of magnitude at low temperatures.

Abstract: In this study, stoichiomectric sodium niobate was synthesized by two alkali routes. On the first route, niobium oxide was used as precursor, whereas metallic niobium was the precursor used for NaNbO₃ synthesis.

Abstract: Ferroelectric niobates with tetragonal tungsten bronze TTB-type structure have shown great technology interest for application in pyroelectric detectors and piezoelectric transducers, sensor/filter in composite. The synthesis and structural characterization of the solid solution of iron doped potassium strontium niobate with stoichiometry KSr₂(Fe_0.25Nb_4.75)O_15-δ prepared by modified polyol method were carried out. The highest crystallinity was obtained for the precursor powder calcined at 1150oC for 10 hours in oxygen atmosphere. The structural characterization was performed by X-Ray diffraction. The average crystallite size obtained was equal to 25 nm. The structural parameters were determined by Rietveld method giving a tetragonal system with space group P4bm. The spontaneous polarization of the solid solution was calculated, being equal to 39.46 μC.cm^-2. Polihedra distortion and its correlation with niobium off-center are discussed.

Abstract: Ferroelectric semiconductors oxides with tetragonal tungsten bronze TTB-type structure doped with rare earth ions have been investigated in recent years due their promising optical properties. K₂Nd_(1-x)Eu_xNb₅O₁₅ nanopowders, where x = 0, 0.0025, 0.025, 0.05 and 0.1 were synthesized by the modified polyol method and characterized by X-ray diffraction and diffuse reflectance spectroscopy UV-Vis. Single phase and crystalline powders of tetragonal symmetry with non-centrosymmetric space group P4bm were obtained. The diffuse reflectance spectra were similar to the profile of a semiconductor material with the presence of thin transitions of Eu³⁺ in europium-doped K₂NdNb₅O₁₅, as well as the transitions of Nd³⁺. The method of Kubelka-Munk was used for the estimation of bandgap energy. The values around 3.7 eV showed small variation with the concentration of Eu³⁺ ions in the K₂NdNb₅O₁₅ host structure. The transitions were identified as direct ones type.

Abstract: Tetragonal Tungsten Bronze structure TTB-type structure has attracted interest by the high anisotropy of the crystal structure. The dielectric characterization of iron-doped niobate of TTB-type structure, with stoichiometry KSr₂(Fe_0.25Nb_4.75)O_15-δ, prepared by Modified Polyol Method was investigated. Nanocrystalline single phase powders were obtained after calcination of the precursor powder at 1150 °C for 10 hours in an oxygen atmosphere. The dielectric characterization was performed by impedance spectroscopy, from room temperature to 600 °C, in the frequency range of 5 Hz to 13 MHz. The permittivity values obtained for KSr₂(Fe_0.25Nb_4.75)O_15-δ showed superior to the permittivity values of the KSr₂Nb₅O₁₅ host structure in all temperature range investigated. At room temperature, the permittivity values (2100) of KSr₂(Fe_0.25Nb_4.75)O_15-δ is two times the permittivity values of KSr₂Nb₅O₁₅. The substitution of niobium cation by iron cation in the KSr₂Nb₅O₁₅ host structure showed a suppression of the ferroelectric (P4bm) → paraelectric (P4/mbm) phase transition.

Abstract: The composites have a great use in practical application. In common, the phases in composite have different relative dielectric constant and in order to reveal how the phases with different permittivity affect the composite’s dielectric properties, the experiments were carried out using inorganic and organic composite with different dielectric constant phases to make that clear. The barium niobate-based SiO₂ system glass–ceramic and fillers-epoxy resin composites were chosen, and the dielectric properties were tested to compare the difference of those composites. The results show that the existence of high dielectric constant phases in composites can improve the permittivity of composites and make the composites present ferroelectric properties, while the dielectric loss can also increased, and the difference in dielectric constant of the phases can decrease the dielectric breakdown strength.

Abstract: Hydroxyapatite is a bioceramic material of great interest for use as bone substitute because of its similarity with the composition of biological apatite. Cationic and anionic substitutions in the apatite structure have been made in order to optimize the synthesis and accelerate the process of bone repair. In the present study, niobate apatite was synthesized by a patented aqueous precipitation method. The bioactivity of the samples was assessed by X-ray diffraction analyses (XRD), energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy with field emission gun (FEG-SEM; FEI Quanta FEG 250) in the samples before and after an incubation period in simulated body fluid. The results showed that after 3 days a bone-like apatite coating was formed onto the niobate apatite surface. A peculiar morphology comprised by nanosized wires was also observed on the niobate apatite surface.

Abstract: Modification of surface morphology and composition of niobophosphate glass and glass-ceramic in the system P₂O₅-CaO-Nb₂O₅-Na₂O was investigated. Amorphous, partly crystalline and crystalline samples were prepared in shape of pellet with diameter 10 mm. Samples were treated in 1 and 5 days in citric acid at 37°C and 3h in boiling HCl. Surface morphology before and after treatment was investigated by scanning electron microscopy (SEM), phase composition by X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). Glass sample was smooth, after treatment in citric acid it had cracks and pit-holes on the surface. Partly crystalline sample was smooth and contained inclusions of crystalline Ca₃(PO₄)₂ and Ca₂P₂O₇. Platy crystals of Ca₃(PO₄)₂ dissolved first, than bulky Ca₂P₂O₇. Crystalline samples contained Na₄Nb₈P₄O₃₂ and calcium phosphate phases; surface was rough before and after treatment. EDX showed that amount of phosphorous is proportional to amount of calcium and inversely proportional to amount of sodium and niobium in glass-ceramic samples.

Abstract: Chemical bonding effects have been discussed for anhydrous niobates and tantalates using binding energy differences Δ_Nb = (BE O 1s – BE Nb 3d_5/2) and Δ_Ta = (BE O 1s – BE Ta 4f_7/2) as key parameters to relate structural and electronic properties of these dielectric oxides. Such known photocatalytic materials as K₄Nb₆O₁₇, Sr₂Nb₂O₇, Ta₂O₅, LiTaO₃, NaTaO₃, NaTa₂O₆ and Sr₂Ta₂O₇ are considered in particular.

Abstract: The layer structured niobate Cs4Nb6O17•3H2O has been prepared in a solid state reaction using Cs2CO3 and Nb2O5 at 1073 K. Ion exchange reactions of Cs+ in the interlayer space were studied in aqueous solutions. The single phases of lithium, sodium and hydrogen ion exchange products were obtained and they also contained interlayer water. The interlayer water of the lithium ion exchange product was removed by heating at 453 K in a vacuum. The resulting niobate Cs0.26Li3.01H0.73Nb6O17 was evaluated for its use as the cathode in a rechargeable lithium battery. The cathode exhibited discharge and charge capacities of 120 and 110 mAhg-1 for the first cycle in the voltage range of 1.5 – 4.2 V. The amount of Li+ intercalated and deintercalated were 4.0 and 3.6 of the formula unit, respectively.