Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Microwave dielectric properties of the (1 − x)Mg2TiO4-xCaTiO3-y wt.% ZnNb2O6 ceramics system

Composite ceramics based on (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12-0.16, y = 0-8) were prepared by a conventional mixed-oxide route. Zn²⁺ partially replaced Mg²⁺ in Mg₂TiO₄ and formed the spinel-structured (Mg_1−δZn_δ)₂TiO₄ phase. Nb²⁺, is known to be solid soluble in CaTiO₃, was found to change its shape from cubic to pliable. A bi-phase system (Mg_1−δZn_δ)₂TiO₄ and CaTiO₃ exhibited in all samples, where a small amount of second phase Mg_1−δZn_δTiO₃ was also detected. The microwave dielectric properties of specimens were strongly related to ZnNb₂O₆ and CaTiO₃ content. As y increased, ?_r and τ_f decreased, however, Q × f decreased to a minimum value and started to increase thereafter. It was also found that ?_r and τ_f increased and Q × f decreased with increasing x. The optimized microwave dielectric properties with ?_r = 18.37, Q × f = 31,027 GHz (at 6 GHz), and τ_f = 0.51 ppm/°C were achieved for (1 − x)Mg₂TiO₄-xCaTiO₃-y wt.% ZnNb₂O₆ (x = 0.12, y = 4) sintered at 1360 °C for 6 h.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Synthesis of mono-dispersed spherical Nd:Y2O3 powder for transparent ceramics

Transparent Nd:Y₂O₃ ceramic was obtained by sintering mono-sized spherical powder. The powder was prepared by homogeneous precipitation method in aqueous media using urea to regulate the pH. The structure and morphology of the powder were investigated by TG-DTA, XRD, SEM and IR spectrum. The effect of aging temperature, time, and the concentration of urea, [Y³⁺], and [Nd³⁺] were investigated. Results showed that the obtained precursor was R₂(OH)CO₃·H₂O (R = Y, Nd), and the least size of mono-sized spherical yttria particles was 72 nm by a microwave oven method after calcinations at 850 °C for 4 h. After dry press and CIP, the particles accumulated closely, and no defects can be detected in the green body.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Synthesis and optical characterization of novel Sr3Ga2O6:Eu phosphor

Alkaline earth metal gallets have been identified as an important ceramic material. The crystal chemistry of many of these gallets is well explored; however, very rare studies regarding optical properties of rare earth (RE) ions doped in such gallets, particularly in Sr₃Ga₂O₆ host, have been carried out. The present study reports on synthesis and characterization of novel Sr₃Ga₂O₆:Eu³⁺ phosphors. The phosphors have been synthesized using a conventional solid state reaction method. Crystal structure, morphology and luminescence properties (excitation, emission and CIE coordinate) of these phosphors have been studied as a function of sintering temperature and Eu³⁺ concentration. X-ray diffraction study reveals that the phosphor sintered at low temperature (900 °C) contains an impurity phase which is removed at higher sintering temperatures and results into cubic crystalline phase of Sr₃Ga₂O₆. Particle size of the phosphor increases with an increase in sintering temperature which results to a red shift in the peak position of excitation band lying in a broad range from 250 to 370 nm. Optimum emission intensity is attained for 0.12 mol% concentration of Eu³⁺ ions; above this concentration, a quenching in emission intensity is observed.     

On the LiCo2/3Ni1/6Mn1/6O2 positive electrode material

LiCo_2/3Ni_1/6Mn_1/6O₂ layered oxide was synthesized by the combustion method that led to a crystalline phase with good homogeneity and low particles size. The structural properties of the prepared positive electrode material were investigated by performing XRD Rietveld refinement. Practically no Li/Ni mixing was detected evidencing that the studied compound adopts almost an ideal α-NaFeO₂ type structure. The Li||LiCo_2/3Ni_1/6Mn_1/6O₂ cell showed a discharge capacity of 199 mAh g⁻¹ when cycled in the 2.7–4.6 V potential range while the best cycling performances were recorded when the upper cut off is fixed at 4.5 V. Structural changes in Li_xCo_2/3Ni_1/6Mn_1/6O₂ with lithium electrochemical de-intercalation were studied using X-ray diffraction. This study clearly shows the existence of a solid solution domain in the 0.1 < x < 1.0 composition range while for x = 0.1, a new phase appears explaining the decrease of the electrochemical performance when the cell is cycled at high upper cut off voltage.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

XANES study of polyaniline-V2O5 and sulfonated polyaniline-V2O5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline-V₂O₅ and sulfonated polyaniline-V₂O₅, which have a higher charge capacity than the V₂O₅ xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO₅ units, and this was attributed to the intimate contact between V₂O₅ and the polymers, and to some flexibility of the VO₅ square pyramids due to the low range order of the nanocomposites.     

A facile molten salt route to K2Nb8O21 nanoribbons

Single-crystalline K₂Nb₈O₂₁ nanoribbons with width of 100–500 nm and thickness of ca. 30 nm, and length up to tens of micron, have been successfully synthesized by simply calcining Nb₂O₅ powders in molten KCl, and characterized with XRD, SEM, SEM, HRTEM and selected-area electron diffraction technique. The growth direction of the obtained K₂Nb₈O₂₁ nanoribbons was determined to be the 〈1 0 0〉 crystallography direction.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Effect of Y2O3 and Er2O3 co-dopants on phase stabilization of bismuth oxide

Bi₂O₃ compositions were prepared to investigate the effect of rare earth metal oxides as co-dopants on phase stability of bismuth oxide. Compositions containing 9-14 mol% of Y₂O₃ and Er₂O₃ were synthesized by solid state reaction. The structural characterization was carried out using X-ray powder diffraction. The XRD results show that the samples containing 12 and 14 mol% total dopants had cubic structure, whereas the samples with lower dopant concentrations were tetragonal. Comparing the lattice parameters of the cubic phases of (Bi₂O₃)_0.88(Y₂O₃)_0.06(Er₂O₃)_0.06 and (Bi₂O₃)_0.86(Y₂O₃)_0.07(Er₂O₃)_0.07 revealed that lattice parameter decreases by increasing the dopant concentration. The XRD pattern and the powder density results indicated the formation of solid solution in the studied systems. After annealing samples with cubic phase at 600 °C for various periods of time, phase transformation to tetragonal and rhombohedral occurs.     

Preparation of nanocrystalline Sr3Al2O6 powders via citric acid precursor

Sr₃Al₂O₆ was synthesized via citric acid precursor. The effects of the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of Sr₃Al₂O₆ were investigated. Increasing the CA/M promoted the formation of Sr₃Al₂O₆. Single-phase and well-crystallized Sr₃Al₂O₆ was obtained from the CA/M = 1, CA/M = 2 and CA/M = 4 precursor at temperature 1200 °C, 1100 °C and 900 °C, respectively. Differential thermal analysis and thermogravimetric (DTA/TG), X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. Sr₃Al₂O₆ nanoparticles with a diameter of about 50-70 nm were obtained.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

Mixed oxides Ca2Fe2O5 and Ca2Co2O5 as anode materials for Li-ion batteries

The electrochemical performance of mixed oxides, Ca₂Fe₂O₅ and Ca₂Co₂O₅ for use in Li-ion batteries was studied with Li as the counter electrode. The compounds were prepared and characterized by X-ray diffraction and SEM. Ca₂Fe₂O₅ showed a reversible capacity of 226 mAh/g at the 14th cycle and retained 183 mAh/g at the end of 50 cycles at 60 mA/g in the voltage window 0.005-2.5 V. A reversible capacity in the range, 365-380 mAh/g, which is stable up to 50 charge-discharge cycles is exhibited by Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V and at 60 mA/g. This corresponds to recycleable moles of Li of 3.9±0.1 (theoretical: 4.0). Significant improvement in the cycling performance and attainable reversible capacity were noted for Ca₂Co₂O₅ on cycling to an upper cut-off voltage of 3.0 V as compared to 2.5 V. Coulombic efficiency for both compounds is >98%. Electrochemical impedance spectroscopy (EIS) data clearly indicate the reversible formation/decomposition of polymeric surface film on the electrode surface of Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V. Cyclic voltammetry results compliment the galvanostatic cycling data.     

Electrochemical reduction behavior of U3O8 powder in a LiCl molten salt

The reduction path of the U₃O₈ powder vol-oxidized at 1200 °C has been determined by a series of electrochemical experiments in a 1 wt.% Li₂O/LiCl molten salt. Various reaction intermediates are observed by during electrolysis of U₃O₈. The formation of the metallic uranium is caused from two different reduction paths, a direct reduction of uranium oxide and an electro-lithiothermic reduction. As the uranium oxide is converted to the metallic uranium, the lithium metal is more actively formed in the cathode basket. The reducibility of the rare earth oxides with the U₃O₈ powder has been tested by constant voltage electrolysis. The results suggest the advanced vol-oxidation could lead to the enhancement in the reducibility of the rare earth fission products.     

Accommodation of impurities in α-Al2O3, α-Cr2O3 and α-Fe2O3

Atomic scale computer simulation was used to predict the mechanisms and energies associated with the accommodation of aliovalent and isovalent dopants in three host oxides with the corundum structure. Here we consider a much more extensive range of dopant ions than has previously been the case. This enables a rigorous comparison of calculated mechanism energetics. From this we predict that divalent ions are charge compensated by oxygen vacancies and tetravalent ions by cation vacancies over the full range of dopant radii. When defect associations are included in the model these conclusions remain valid. At equilibrium, defects resulting from extrinsic dopant solution dominate intrinsic processes, except for the largest dopant cations. Solution reaction energies increase markedly with increasing dopant radius. The behaviour of cluster binding energies is more complex.