Oxygen Coefficient of Uranium Oxides and Current Efficiency as Influenced by Electrolysis Parameters and Composition of Molten Salt Electrolytes in the Na2WO4-Na2W2O7-UO2WO4 System

The oxygen coefficient of uranium oxides, their structure, and current efficiency were studied as influenced by the electrolyte composition, deposition potential, and temperature of electrolysis. On passing from Na₂WO₄-UO₂WO₄ binary system to lower-melting ternary systems the dependences of the oxygen coefficient in the cathodic product on the electrolyte composition and electrolysis parameters remain essentially similar. Significant deviation of the experimental current efficiency with respect to uranium oxides from the theoretical value suggests significant chemical interaction between the cathodic product and electrolyte. The corrosion rate increases and the current efficiency decreases with increasing temperature and concentration of W₂O₇ ^2- ions. The structure of the resulting cathodic deposits is predominantly determined by their specific electrical conductivity, which is a function of the chemical composition of the electrolyte. The dendrite structure is typical for higher oxides.     

Sequential recovery of uranium oxides by electrolysis of M2WO4-M2W2O7-UO2WO4 melts (M = Li, Na, K, Cs)

The effect of the electrolyte composition and solvent salt cation on the oxygen coefficient of the cathode product (O/U atomic ratio) and on the main characteristics of potentiostatic electrolytic deposition of UO₂ (cathode current efficiency, current density, product deposition rate) in sequential recovery of uranium oxides from electrolytes of the system M₂WO₄-M₂W₂O₇-UO₂WO₄ (M = Li, Na, K, Cs) in air was examined. The dependence of the oxygen coefficient of the cathode product on the electrolyte composition and solvent salt cation does not differ essentially from that observed previously in short experiments with low melt depletion of U. The current efficiency, initial current density, and product deposition rate decrease with the electrolyte depletion of U. The results obtained are satisfactorily accounted for in terms of the model of ionic composition of uranyl-containing oxide salt electrolytes, based on the concept of complexation and stepwise solvolysis of uranyl ions, taking into account formation of lower valence forms of U due to chemical corrosion of the cathode product.     

Liquidus relations in the Li2WO4-Na2WO4-CaWO4 system

Phase relations in the Li₂WO₄-Na₂WO₄-CaWO₄ system were studied by physicochemical analysis. The liquidus surface was mapped out, and the compositions and temperatures of invariant points were determined.     

Management of composition cathodic products in the electrolysis of molybdenum‐, tungsten‐ and carbon‐ bearing halogenide‐oxide and oxide melts

It is shown that the cathode products of electrolysis of melts based on a eutectic mixture of sodium chloride and lithium fluoride as well as melts based on sodium tungstate in which dissolved oxides of molybdenum (VI) or tungsten (VI), molybdate, tungstate and lithium or sodium carbonates are molybdenum, tungsten, their bronzes and carbides, carbon. It is established that the phase composition of electrolysis products is determined by the concentration of carbonate in the melt. Particular conditions of plating coating of molybdenum and tungsten carbides on carbon, nickel and copper materials are determined. Molybdenum carbide coatings of electrolyte Na₂WO₄–Li₂MoO₄–Li₂CO₃ are deposited at equality (within 2.5 mol%) of concentrations molybdate and lithium carbonate. However, their concentrations should not exceed 10 mol%. At lower concentrations of molybdate in the precipitate are detected carbon, molybdenum, molybdenum carbide, and at high concentrations – molybdenum oxides. At lower concentrations of carbonate in the sediment dominates molybdenum and at large concentrations mainly free carbon is released. More affordable industrial reagent‐source of molybdenum is its oxide.     

Influence of cations on the vibrational spectra and structure of [WO<Subscript>4</Subscript>] complexes in molten tungstates

The vibrational spectra and structure of isolated [WO₄]^2– anions in molten alkali, alkaline-earth, rare-earth, Pb, Bi, Zn, Sc, and Cd tungstates are studied by high-temperature Raman spectroscopy. The degenerate modes of the [WO₄]^2– anion in K-Gd tungstate melts are found to be split, which is interpreted in terms of the symmetry of the cation environment of the [WO₄]^2– anion in the melts. The frequency of the fully symmetric stretching mode (₁) of the [WO₄]^2– group in molten tungstates is examined in relation to the polarizing power P and electronegativity of the cation. The origin of the nonmonotonic variation of ₁ with P and for the tungstate melts studied is discussed.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 493–502.Original Russian Text Copyright © 2005 by Voronko, Sobol.     

Morphology of Bi2WO6 powders obtained in the presence of fused salts

Bismuth tungstate (Bi₂WO₆) powders are formed from constituent oxides in the presence of molten chloride and sulphate salts, i.e. 0.5 NaCl-0.5 KCl and 0.365 Na₂SO₄-0.635 Li₂SO₄, in the temperature range from 650 to 900° C. The morphology of powders so obtained depends on the salt system and the experimental conditions. Plate-like particles are obtained by use of the chloride salts. The particle size of the materials is about 5 and 100 m when prepared below and above the liquidus temperature in the pseudo-binary Bi₂WO₆-chloride salt system respectively. The morphology of the particles obtained by using sulphate salts is complicated. Particles having characteristics of orthorhombic symmetry are obtained in the initial period of the reaction, but they become spheroidal with further heating, finally approaching an oblate form. Mechanisms of morphology changes are discussed and it is concluded that the chief difference in the salt effect is the strong stabilizing effect of (010) in the chloride salts.     

Preparation and characterization of rapidly quenched glasses in the systems R2O—WO3 (R=Li,Na, K)

In the systems R₂O-WO₃ (R=Li, Na, K), the glass-forming region was extended by use of the rapid quenching technique combining a thermal-image furnace with a twin roller. The infra-red spectra revealed that the Li₂WO₄ glass contains no condensed macro-anions but only discrete ions of Li⁺ and WO ₄ ^2– . The glass transition temperature (T _g) and the crystallization temperature (T _c) were determined.T _g is affected very slightly by the type of alkali cations, whereas it is decreased with increase in the alkali oxide content. More than oneT _g was observed in Li₂O-WO₃ glasses, which is due to the formation of metastable phases. The conductivity of glasses containing the same amount of alkali oxide (of different types) increased with decrease in the radius of the alkali cation. Electrochromism was observed in the glasses with R=Li, Na; the electric field required for colouration is lower than that in WO₃ films. The electro-colouration is due to H_xWO₃ formed in the glass matrix by the double injection of protons and electrons.     

Growth and Luminescent Properties of NaGd(WO4)2:Yb3+ Crystals

Data are presented on the spectroscopic properties of Yb³⁺-activated NaGd(WO₄)₂, a disordered scheelite-like tungstate potentially attractive as a gain medium. NaGd(WO₄)₂:Yb³⁺ crystals are grown by the Czochralski technique. The polarized absorption and luminescence spectra and the luminescence decay kinetics of oriented samples with different Yb³⁺ concentrations are studied at 300 K. The gain coefficients are calculated for different populations of the upper lasing level 2F _5/2 of the Yb³⁺ ion.     

Thermal Analysis of the Ternary System Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>-WO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

Thermal analysis results indicate that the liquidus surface of the Li₂WO₄-WO₃-Li₂B₄O₇ system consists of the primary phase fields of Li₂WO₄, Li₂B₄O₇, WO₃, Li₂WO₄ · WO₃ (congruent melting), 3Li₂WO₄ · 2Li₂B₄O₇ (congruent melting), and Li₂WO₄ · 3WO₃ (incongruent melting). Low-melting-point compositions are selected that are potentially attractive for the low-temperature synthesis of lithium tungsten bronze powders.     

Enhanced thermoelectric properties of WO3 by adding SnO2

Tungsten trioxide (WO₃) doped with stannic oxide (SnO₂) was prepared by a conventional mixed oxide processing route. The microstructure of the SnO₂-added WO₃ ceramics samples were investigated by X-ray diffraction and scanning electron microscopy. The grain size and porosity decreased with the increasing SnO₂ content. The sign of the Seebeck coefficient for all samples was negative over the entire temperature range, i.e., n-type conduction. The addition of SnO₂ to WO₃ led to an increase in both the electrical conductivity and the absolute value of the Seebeck coefficient. This indicates that the power factor was significantly enhanced by adding SnO₂ to WO₃. The thermoelectric power factor was maximized to 7.77 μW m^?1 K^?2 at 1,023 K for the 10.0 mol% sample.     

WO3 nano-ribbons: their phase transformation from tungstite (WO3·H2O) to tungsten oxide (WO3)

Tungsten oxide (WO₃) nano-ribbons (NRs) were obtained by annealing tungstite (WO₃·H₂O) NRs. The latter was synthesized below room temperature using a simple, environmentally benign, and low cost aging treatment of precursors made by adding hydrochloric acid to diluted sodium tungstate solutions (Na₂WO₄·2H₂O). WO₃ generates significant interests and is being used in a growing variety of applications. It is therefore important to identify suitable methods of production and better understand its properties. The phase transformation was observed to be initiated between 200 and 300 ^°C, and the crystallographic structure of the NRs changed from orthorhombic WO₃·H₂O to monoclinic WO₃. It was rigorously studied by annealing a series of samples ex situ in ambient air up to 800 ^°C and characterizing them afterward. A temperature-dependent Raman spectroscopy study was performed on tungstite NRs between minus 180 and 700 ^°C. Also, in situ heating experiments in the transmission electron microscope allowed for the direct observation of the phase transformation. Powder X-ray diffraction, electron diffraction, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize precisely this transformation.     

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu,and Y), phase/morphology evolution,and photoluminescence

Abstract

Hydrothermal reaction of Ln nitrate and Na₂WO₄ at pH=8 and 200 °C for 24 hours, in the absence of any additive, has directly produced the scheelite-type sodium lanthanide tungstate of NaLn(WO₄)₂ for the larger Ln³⁺ of Ln=La-Dy (including Y, Group I) and an unknown compound that can be transformed into NaLn(WO₄)₂ by calcination at the low temperature of 600 °C for the smaller Ln³⁺ of Ln=Ho-Lu (Group II). With the successful synthesis of NaLn(WO₄)₂ for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature- and time-course phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO₄)₂ and NaTb(WO₄)₂, including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.     

New rutile solid solutions,Ti1−4xLixM3xO2: M = Nb,Ta, Sb

Three extensive new rutile solid solution series have been prepared in which Ti⁴⁺ is replaced by a combination of Li⁺ and a pentavalent cation: Nb⁵⁺, Ta⁵⁺, Sb⁵⁺. The formulae are Ti_1?4xLi_xM_3xO₂: 0 < x ? 0.15, M = Ta; 0 < x ? 0.17, M = Nb; 0 < x ? 0.12, M = Sb. The solid solutions were characterised by X-ray powder diffraction and density measurements. In addition to the rutile solid solutions, LiNb₃O₈ forms a limited range of solid solutions, Li_1?yNb_3?3yTi_4yO₈: 0 < y ? 0.06.     

A New Type of Li‐Rich Rock‐Salt Oxide Li2Ni1/3Ru2/3O3 with Reversible Anionic Redox Chemistry

Li‐rich oxide cathodes are of prime importance for the development of high‐energy lithium‐ion batteries (LIBs). Li‐rich layered oxides, however, always undergo irreversible structural evolution, leading to inevitable capacity and voltage decay during cycling. Meanwhile, Li‐rich cation‐disordered rock‐salt oxides usually exhibit sluggish kinetics and inferior cycling stability, despite their firm structure and stable voltage output. Herein, a new Li‐rich rock‐salt oxide Li₂Ni_1/3Ru_2/3O₃ with Fd‐3m space group, where partial cation‐ordering arrangement exists in cationic sites, is reported. Results demonstrate that a cathode fabricated from Li₂Ni_1/3Ru_2/3O₃ delivers a large capacity, outstanding rate capability as well as good cycling performance with negligible voltage decay, in contrast to the common cations disordered oxides with space group Fm‐3m. First principle calculations also indicate that rock‐salt oxide with space group Fd‐3m possesses oxygen activity potential at the state of delithiation, and good kinetics with more 0‐TM (TM = transition metals) percolation networks. In situ Raman results confirm the reversible anionic redox chemistry, confirming O^2?/O^? evolution during cycles in Li‐rich rock‐salt cathode for the first time. These findings open up the opportunity to design high‐performance oxide cathodes and promote the development of high‐energy LIBs.     

Raman spectra and phase transformations of the MLn(WO4)2 (M = Na, K; Ln = La, Gd, Y, Yb) tungstates

The 300-K crystal structure, phase transformations, and melting of the MLn(WO₄)₂ (M = Na, K; Ln = La, Gd, Y, Yb) tungstates and the structure of their melts were studied by Raman spectroscopy.     

A new microwave dielectric ceramics for LTCC applications: Li2Mg2(WO4)3 ceramics

A novel microwave dielectric ceramics Li₂Mg₂(WO₄)₃ (LMW) for low-temperature co-fired ceramics (LTCC) application were prepared by the conventional solid-state sintering method. Densification, phases, microstructure and microwave dielectric properties of the Li₂Mg₂(WO₄)₃ ceramics were investigated. The optimal sintering temperature of dense Li₂Mg₂(WO₄)₃ ceramic approximately ranges from 825 to 875 °C for 3 h. The ceramic specimens fired at 875 °C for 3 h exhibits excellent microwave dielectric properties: ε _r  = 7.72, Q × f = 29,600 GHz (f = 6.0 GHz), and τ _f  = ?15.5 ppm/°C. Moreover, the Li₂Mg₂(WO₄)₃ ceramics has a chemical compatibility with Ag during cofiring, which makes it a promising ceramic for LTCC technology application.     

The effects of Na2WO4 concentration on the properties of microarc oxidation coatings on aluminum alloy

Alumina coating, approximately 30 μm, was deposited on an LY12 Al alloy substrate using a microarc oxidation (MAO) process in a H₃BO₃–KOH electrolyte solution with the Na₂WO₄ addition varying from 0 to 6 g/l. The MAO process was studied by measuring the voltage as a function of time. The coating layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with respect to the phases and microstructures and by measuring microhardness and wear resistance. The results show that the concentration of Na₂WO₄ has direct effects on the behavior of MAO process and the quality of the MAO coatings as well. The final phases in the coating were found to be α-Al₂O₃ and both γ-Al₂O₃ and a small amount of W. Without an addition of Na₂WO₄, the MAO coating process could not successfully proceed. With increasing the Na₂WO₄ concentration in the electrolyte, the working voltage at the microarc discharge stage decreased, the thickness and the content of α-Al₂O₃ phase in the coating both reduced. The microhardness and the wear resistance were both enhanced as the content of α-Al₂O₃ phase increased.     

The formation of WO3 nanorods using the surfactant-assisted hydrothermal reaction

Monoclinic tungsten oxide (WO₃) nanorods were grown using the hydrothermal method on a seeded W foil. The seed layer was formed by thermal oxidation of W foil at 400°C for 30 min. Cetyltrimethylammonium bromide (CTAB) or hexamethylamine (HMT) was used in the reactive hydrothermal bath, along with sodium tungstate dihydrate (Na₂WO₄.2H₂O) and hydrochloric acid (HCl). The concentration of CTAB was varied from 0.01 M to 0.07 M and the concentration of HMT was varied from 0.01 M and 0.05 M. The result showed that CTAB-assisted hydrothermal reaction produced WO₃ nanorods with 4–7 nm diameter, and provided that CTAB concentration was less than 0.07 M. WO₃ nanorods could not be obtained when CTAB concentration was 0.07 M. Columnar structured WO₃ was produced with the presence of HMT in the hydrothermal bath. This was due to decomposition of HMT to form hydroxyl ions (OH^?) that inhibited the growth of nanorods. Cyclic voltammetry (CV) analysis showed better electrochromic property of WO₃ nanorods compared to columnar structured WO₃.     

Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al₂(WO₄)₃ was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) Å, and V = 1039.5(3) (Å)³. The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850°C. The average linear NTE coefficient (₁), in this temperature range, was –1.5 × 10^–6 K^–1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO₄ and WO_3–x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO_3–x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400°C, 3 GPa/900°C and 1.8 GPa/1030°C, respectively. The detailed XRD studies of the products are presented here.     

Raman spectroscopy study of the phase transformations of LiB3O5 and Li2B4O7 during heating and melting

The incongruent melting of LiB₃O₅ (LBO) single crystals and transformations of the boron-oxygen groups in glasses and melts with the compositions Li₂O · 2B₂O₃ and Li₂O · 3B₂O₃ have been studied by Raman spectroscopy. The vapor over superheated melts of these condensed lithium borates has been shown to contain [BO₂] radicals.