Showing papers on "Tungstate published in 2000"

Stimulated Raman scattering in alkaline-earth tungstate crystals

Abstract: Results on spontaneous Raman spectroscopy of alkaline-earth tungstate crystals are presented. The frequency of the modes active in stimulated Raman scattering (SRS) modes was found to increase and their width to decrease with increasing cation radius and mass in the series of alkaline-earth (Ca, Sr, Ba) tungstates. High peak values of the Raman scattering cross section for barium and strontium tungstate crystals — new media offering promise for SRS — were predicted and observed in the experiments. Laser experiments on the SRS threshold showed that the SRS gain for the barium tungstate crystal was close to the record value observed for the barium nitrate crystal, which is extensively used in SRS.

Tungstate versus Molybdate Adsorption on Oxidic Surfaces: A Chemical Approach

Abstract: The differences between the chemistry of tungstate adsorption on oxidic supports and that of molybdate (viz. γ-Al2O3, TiO2 and amorphous silica alumina (ASA)) were studied by measuring equilibrium adsorption isotherms at various conditions (pH, (co)-adsorption of tungstate and molybdate), by determining which part was reversibly adsorbed, and by structurally characterizing them using FTIR and Raman spectroscopy. It can be concluded that most of the tungstate reacts irreversibly with acidic and neutral OH groups, and the other part adsorbs reversibly by electrostatic interactions with protonated OH groups, whereas molybdate irreversibly reacts in a reaction with the basic OH groups. However, as soon as these groups are protonated, molybdate also starts to reversibly adsorb by electrostatic interactions. In addition, molybdate adsorbs on coordinatively unsaturated sites whereas tungstate does not.

Tungsten, Tungsten Alloys, and Tungsten Compounds

Abstract: The article contains sections titled: 1 Introduction 2 Properties 21 Physical Properties 22 Chemical Properties 3 Raw Materials 31 Natural Resources 32 Tungsten Scrap 4 Production 41 Mining and Ore Beneficiation 42 Pretreatment of Ore Concentrates and Scrap 43 Hydrometallurgy 431 Digestion 432 Purification 433 Conversion of Sodium Tungstate Solution to Ammonium Tungstate Solution 434 Crystallization of Ammonium Paratungstate (APT) 44 Production of Tungsten Oxides 45 Production of Tungsten Metal Powder 46 Production of High-Purity Tungsten Metal (99999 - 999999%) 47 Powder Metallurgy (PM) 48 Metal Injection Molding 481 MIM Process Overview 482 General Guidelines 483 MIM of Tungsten, Tungsten Alloys, Tungsten–Copper Composites, Tungsten Heavy Alloy, and Cemented Carbide 49 Additive Manufacturing of Tungsten and Cemented Carbides (WC–Co) 410 Fabrication of Wrought PM Tungsten 4101 Shaping–Mill Products 4102 Mechanical Bonding of Tungsten to Tungsten and Other Metals 411 Surface Treatment 412 Melting 5 Tungsten Alloys 51 Single-Phase Solid-Solution Alloys 52 Multiphase Alloys 521 Tungsten Heavy Metals 522 Tungsten–Copper and Tungsten–Silver Composites 523 Non-Sag Tungsten 524 Alloys with Oxide Dispersions 525 Porous, Infiltrated Tungsten 6 Uses of Tungsten 61 Tungsten and Tungsten Alloys 62 Cemented Carbides (WC–Co) 63 Tungsten Coatings 7 Tungsten in Melting Metallurgy of Steel and Superalloys 71 Tungsten in Steel 72 Tungsten in Superalloys 73 Master Alloys 731 Ferrotungsten 732 Tungsten Melting Base 733 Master Alloys for Superalloys 74 Production of Master Alloys 741 Production of Ferrotungsten 742 Production of Tungsten Melting Base 743 Production of Master Alloys for Superalloys 8 Tungsten Compounds and Their Application 81 Tungsten Chemistry 82 Aqueous Solutions of Tungsten 83 Intermetallic Compounds 84 Compounds with Nonmetals 841 Tungsten–Boron Compounds 842 Tungsten–Carbon Compounds 843 Tungsten–Silicon Compounds 844 Tungsten–Group 15 Compounds 845 Tungsten–Oxygen Compounds 846 Tungsten–Chalcogenide Compounds 847 Tungsten–Halogenide Compounds 9 Tungsten in Catalysis 10 Tungsten Recycling 101 Direct Recycling 102 Semi-Direct Methods 103 Hydrometallurgy 104 Melting Metallurgy 11 Analysis 111 Raw Materials 112 High Purity Intermediate Products, Tungsten Powder and Sintered Tungsten Metal 113 Trace Elements in High-Purity Tungsten Metal 12 Economic Aspects 121 Production 122 Consumption 123 Price 13 Toxicology and Occupational Health 131 Toxicokinetics 132 Acute Toxicity 133 Subchronic and Chronic Toxicity 134 Genotoxicity 135 Carcinogenicity 136 Reproductive Toxicity 137 Developmental Toxicity 138 Immunotoxicity 139 Human Biomonitoring Data 1310 Occupational Health 14 Acknowledgements

Molybdenum cofactor amounts in Chlamydomonas reinhardtii depend on the Nit5 gene function related to molybdate transport

Abstract: Strain 21gr from Chlamydomonas reinhardtii is a cryptic mutant defective in the Nit5 gene related to the biosynthesis of molybdenum cofactor (MoCo). In spite of this mutation, this strain has active MoCo and can grow on nitrate media. In genetic crosses, the Nit5 mutation cosegregated with a phenotype of resistance to high concentrations of molybdate and tungstate. Molybdate/tungstate toxicity was much higher in nitrate than in ammonium media. Strain 21gr showed lower amounts of MoCo activity than the wild type both when grown in nitrate and after growth in ammonium and nitrate induction. However, nitrate reductase (NR) specific activity was similar in wild type and 21gr cells. Tungstate, either at nanomolar concentrations in nitrate media or at micromolar concentrations during growth in ammonium and nitrate induction, strongly decreased MoCo and NR amounts in wild-type cells but had a slight effect in 21gr cells. Molybdate uptake activity of ammonium-grown cells from both the wild-type and 21gr strains was small and blocked by sulphate 0·3 mM. However, cells from nitrate medium showed a molybdate uptake activity insensitive to sulphate. This uptake activity was much higher and more sensitive to inhibition by tungstate in the wild type than in strain 21gr. These results suggest that strain 21gr has a high affinity and low capacity molybdate transport system able to discriminate efficiently tungstate, and lacks a high capacity molybdate/tungstate transport system, which operates in wild-type cells upon nitrate induction. This high capacity molybdate transport system would account for both the stimulating effect of molybdate on MoCo amounts and the toxic effects of tungstate and molybdate when present at high concentrations.

New Solution Route to Electrochromic Poly(acrylic acid)/WO3 Hybrid Film

Abstract: Electrochromic tungsten oxide film was fabricated by a new soft chemistry route, in which the transparent conducting glass substrate was successively dip-coated with poly(acrylic acid) (PAA) and ammonium tungstate solutions. The as-coated composite layer of PAA/(NH4)2WO4·nH2O was dried at a low temperature (100 °C), and the ammonium tungstate component was polycondensed by acid treatment in 1 N HCl to form an electrochromic PAA/WO3 film. Transmission electron microscopy showed that the PAA/WO3 film contains regularly dispersed WO3 grains with an average size of ∼5 nm. According to the Auger depth profile analysis, the relative content of W, O, and C was quite homogeneous along the height of film. Depending on the concentration of PAA coating solution (1.0−3.0 wt %), the thickness and tungsten oxide content of the film were found to vary; therefore, the electrode property of the PAA/WO3 layer could easily be controlled. By using a 2.5 wt % PAA solution, an optimum electrochromic function was achieved with ...

Trivalent Ion Conduction in Molybdates Having Sc2(WO4)3-Type Structure

Abstract: A molybdate series with Sc2(WO4)3-type structure, which has been demonstrated to be suitable for trivalent ion conduction, was prepared, and the trivalent ion conducting characteristics of this series were compared with those of the tungstate series. The ionic conductivities of the molybdate series become considerably higher in comparison to those of the tungstate series. Since the ionic radius of Mo6+ is smaller than that of W6+, the oxide anions that form a tetrahedron unit with Mo6+ ion form stronger bonds than those that form a tetrahedron unit with W6+. Stronger hexavalent cation and oxide anion bonding develops more suitable surroundings for trivalent ion migration in the Sc2(WO4)3-type structure. The trivalent ion conducting properties in the molybdate solids were directly characterized by dc electrolysis and EPMA measurements.

Electronic structure and optical properties of CdMoO4 and CdWO4

Abstract: In previous work, we studied the electronic structure of several molybdate and tungstate crystals having the scheelite structure. The wolframite structure is another common form for molybdate and tungstate materials. In this paper, we compare two chemically similar materials that naturally form in the scheelite and wolframite structures—CdMoO4 and CdWO4, respectively. We compare the electronic structure and approximate optical properties of these materials within the framework of density-functional theory. One major difference between the two materials is in their lower conduction bands, which are dominated by the crystal-field-split d bands of the transition metal. For CdMoO4 the Mo ions have an approximately tetrahedral coordination causing the lower conduction band to be twofold degenerate, while for CdWO4 the W ions have an approximately octahedral coordination causing the lower conduction band to be threefold dengenerate.

Polyoxometalates from heteropoly “brown” precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)WIV3WVI17O62Hx]n−

Abstract: Reduction of the α-Keggin anion [Xn+O4W12O36][8 − n]− (X = H22+, B3+, Si4+) by six electrons results in the known tungsten “brown” species [Xn+O4(H2O)3WIV3WVI9O33][8 − n]− in which three W atoms have been reduced from WVI to WIV, forming a metal–metal bonded triad. The WIV atoms have terminal water coordinated in place of terminal oxo groups. Additional tungstate can condense onto these water molecules in aqueous solution between pH = 4 and 6.5 to form the species reported here, [(XO4)WIV3WVI17O62Hx]y−. The boron derivative (X = B3+) is more stable than the metatungstate (X = H22+), both of which have been characterized by elemental analysis, 183W NMR and X-ray crystal structure analysis. Eight additional tungstate groups condense in the form of a partial Keggin structure containing two triads and one dyad which is rotated 60° relative to a hypothetical α-isomer.

Electrodeposition and dissolution of Co–W alloy films

Abstract: Cyclic voltammetric studies were carried out on platinum in alkaline solution containing cobalt sulphate, sodium tungstate, dimethyl sulphoxide and triammonium citrate. Spectral u.v. absorption studies of cobalt citrate complex indicate the species to be [Co L (H2O)3]2+, where L is triammonium citrate. The deposition of cobalt involves a stepwise electron transfer mechanism. The observed cyclic voltammetric data show that the alloy deposition is possibly from cobalt–tungstate complex. The citrate ions and dimethyl sulphoxide hinder the alloy deposition. Dimethyl sulphoxide, which specifically adsorbs on the electrode surface, favours the hydrogen evolution reaction (HER), whereas tungstate hinders the HER. Stripping voltammograms show the existence of cobalt rich alloy phases. X-ray diffraction studies further confirm the phases to be Co3W and Co7W.

Tungstate, molybdate, vanadate base glasses

Abstract: Alkali tungstate, molybdate and vanadate glasses, and telecommunications components embodying such glasses, the compositions of the glasses consisting essentially of 15-70 mol percent of at least one oxide selected from the group consisting of WO3, MoO3 and VO2.5, 0-35 % CrO3, 0-15 % UO3, the total WO3 plus MoO3 plus VO2.5 plus CrO3 plus UO3 being 50-70 %, 20-50 % R2O where R represents at least two elements selected from the group consisting of Li, Na, K, Rb, Cs, Ag and Tl, and optionally containing 0-10 % MO where M is selected from the groups of elements consisting of Ca, Ba, Sr, Mg, Cd, Pb, 0-5 % X2O3 where x is at least one element selected from the group consisting of Al, Ga, In and Bi, 0-5 % of at least one transition metal oxide, 0-15 % P2O5 and/or TeO2 and 0-5 % of a rare earth oxide selected from the lanthanide series.

Tungsten(VI) Extraction Equilibrium in the System Polytungstates—Primene JMT Sulfate—Benzene

Abstract: The study concerns the extraction of tungstate from solutions containing sulphuric acid within a broad range of pH using Primene JMT sulphate in benzene modified with octanol as an extractant. On the basis of the equilibrium composition of both phases, by the determination of metal and sulphate groups concentrations, pH and IR spectrum of the organic phase, it was confirmed that Primene JMT enables the extraction of monomeric and polymeric forms of tungsten anionic species in a broad range of pH (∼2–7.5). The process is similar to that of the extraction of V(V) and Mo(VI), an anion exchange mechanism and a mechanism probably involving an addition can be distinguished.

Precoat steel panel excellent in environmental affinity and processing part corrosion resistance and method for manufacturing the same

Abstract: (57) [Problem] To provide a precoated steel sheet which can obtain excellent properties such as corrosion resistance in a processed part without adding a chromium-based rust preventive pigment in a coating film SOLUTION: This is a precoated steel sheet in which an undercoat film is formed on a surface of a zinc-based plated steel sheet on which a chemical conversion coating film containing silica fine particles and a binder is formed, and an overcoat film is further formed thereon The undercoating film is used as a rust preventive additive component as (a) Ca component: 1 to 30 parts by weight in terms of Ca based on 100 parts by weight of resin solids, (b) SiO 2 and / or silicate: resin S for 100 parts by weight of solids 1 to 35 parts by weight in terms of iO 2 , if necessary, one or more of (c) molybdate, tungstate, etc : 1 to 50 parts by weight based on 100 parts by weight of resin solid content, In addition, the total amount of the rust preventive additives (a) to (c) is 5 to 100 parts by weight based on 100 parts by weight of the resin solid content

Investigation of the kinetics of reduction of manganese tungstate by hydrogen

Abstract: The present work was carried out as a part of a research program studying the reduction kinetics of some transition metal tungstates. Manganese tungstate was reduced by hydrogen under isothermal conditions in the temperature range 883-1183 K. The reaction was found to take place in a single step with MnO and W as the solid products. The activation energy for the reduction of manganese tungstate was evaluated to be 92 kJ mol -1 using the initial reaction rates. This activation energy was compared with the activation energies of the other transition metal tungstates in the light of bond strength of the metal-oxygen bonds.

Tungstate-modified aluminium phosphate: 1. Preparation, characterisation and catalytic activity towards alcohol and cumene conversion reactions

Abstract: A series of tungstate-impregnated aluminium phosphate samples are prepared by varying the tungstate weight percent from 3 to 20 and characterised by XRD and N2-adsorption–desorption method. Surface areas are found to decrease with increase in tungstate loading. However, tungstate-loaded samples exhibit an increasing trend in acidity and catalytic activity. The sample with 10 wt.% tungstate loading shows highest dehydration and dehydrogenation activity for isopropanol and cumene conversion, respectively. The low C2+ hydrocarbon selectivity indicates that the new acid sites formed due to tungstate impregnation is not strong enough to dehydrate methanol to hydrocarbons.

Theoretical predictions and experimental results of the hydrothermal processing of strontium tungstates

Abstract: The hydrothermal processing conditions of strontium tungstate powders were studied by using a thermodynamic model of electrolytic solutions in order to avoid the empirical trial-and-error mode of synthesis. The approach used makes possible to predict the optimum conditions through stability and yield diagrams, relating the equilibrium concentration of all the present species as a function of temperature, pressure, solution pH and input reagent concentrations. The theoretical predictions were verified by experiments at these optimum conditions. The results showed that the thermodynamic model is adequate to predict the synthesis conditions for which the strontium tungstates are stable and can be obtained in a phase-pure form.

Protonation, Oligomerization, and Condensation Reactions of Vanadate(V), Molybdate(VI), and Tungstate(VI)

Abstract: Publisher Summary This chapter discusses protonation, oligomerization, and condensation reactions of vanadate(V), molybdate(VI), and tungstate(VI). The elements vanadium, molybdenum, and tungsten in their high oxidation states form the stable oxyanions [VO4]3-, [MoO4]2-, and [WO4]2-. In aqueous solution, these ions are easily protonated and show a strong tendency to form polyoxoanions by oxygen bridging, and the release of water molecules. Depending on pH and concentration, a great variety of ions can form in each case, giving rise to complex systems of simultaneous equilibria, which have proved difficult to characterize with certainty. The molybdate(VI) system, in particular, needs further clarification regarding the existence of some polyions. More kinetic and thermodynamic data would also help to improve the understanding of these systems and lead to a general inclusive explanation of the mechanism of polyoxoanion formation. In this respect, new information about some structural preferences of the different metals in mixed polyoxoanions is a useful addition to known facts regarding polyoxometalate structures.

Structural phase transition in K1-xCsxDy(WO4)2 single crystals

Abstract: Single crystals of dysprosium potassium (caesium) double tungstates were grown both by typical flux growth and Top Seeded Solution Growth. K 2 W 2 O 7 was used as flux. Potassium tungstate provides broad range of growth temperatures, rather low volatility, and does not introduce additional components into the melt. K 1-x Cs x Dy(WO 4 ) 2 crystals belong to the monoclinic syngony (α-KY(WO 4 ) 2 structure) and has the space group C2/c. Dy ions are situated on the two-fold axes (C 2 ) (which coincide with the crystallographic [010] directions) inside distorted octahedra of oxygen ions. KDy(WO 4 ) 2 is known to undergo a structural phase transition of Jahn-Teller type at 6.38K. The influence of caesium ions on structural phase transition was investigated. Spectroscopic studies of this compound at low temperature have revealed some peculiarities of optical absorption spectra. The ground state of the Dy 3+ ions is split into Kramer's doublets with the first excited doublet lying closely to the ground doublet what determines the thermal behaviour of absorption spectra. The increase of the distance between the Dy 3+ lowest doublets observed at low temperature is interpreted as the result of the cooperative Jahn-Teller effect.

Mercury(II) tungstate from neutron powder data

Abstract: Mercury(II) tungstate powder, HgWO4, was prepared by boiling a mixture of HgO and H2WO4 in water. Rietveld refinements on neutron powder data showed that the monoclinic structure (C2/c) consists of zigzag chains of edge-sharing HgO6 and WO6 octahedra. The Hg atom lies on an inversion centre and the W atom lies on a twofold axis. The Hg atom forms two characteristic short collinear Hg—O bonds.