Showing papers on "Tungstate published in 1993"

Properties of doped and undoped crystals of single domain KTiOAsO4

Abstract: Large single crystals of KTiOAsO4 (KTA) measuring up to 35×31×58 mm3 (along abc) are grown from the pure potassium arsenate self-fluxes and from the tungstate flux using the seeded high-temperature solution growth technique. Small amounts of Fe2O3, Sc2O3, or In2O3 (∼0.2–0.4 wt %) are added to the melt to promote single-domain formation in the flux-grown crystals. We report here the linear optical and nonlinear optical properties of these crystals. We observed an unusually strong optical birefringence increase with Fe2O3 dopant concentration [δ(nz−nx)∼0.017/wt % Fe]. This increase leads to a blue shift in the second-harmonic generation cutoff wavelength of as much as 37 nm for a ∼0.47 wt % Fe-dopant level in KTA.

The crystal structure of wolframite type tungstates at high pressure

Abstract: High pressure / Tungstates / Wolframites / Crystal structure Abstract. Single-crystal structure determinations of wolframite-type struc­ tures were performed using a diamond anvil cell with beryllium gaskets. Unit cell parameters of MgW04 , MnW04 and CdW04 were determined at several pressures up to 8.0 GPa and structure refinements were performed up to 6.0 GPa. The high pressure behaviour is compared with the behaviour of the related scheelite-type structure. All investigated compounds compress anisotropically with the b-axis 1.5 to 2.5 times more compressible than the c-axis and 1.2 to 2.1 times more compressible than the a-axis. In all compounds, the W06 polyhedra are significantly less compressible than the A06 octahedra; the compressions of the latter are comparable with the bulk compressions of the compounds. Anisotropies in the pressure response of the A06 and W06 polyhedra result in the anisotropic unit-cell compression. With increasing pressure the oxygen atomic positions remain constant within one standard deviation, while the heavy atoms positions show a different pressure dependence: a shift of y(Mg) to smaller values and of y(Cd) to higher values, while y(Mn) undergoes little if any change. With increasing pressure the A and W cations approach each other, indicating a tendency of a transition from the wolframite structure to the scheelite structure, where the A and W cations lie in the same plane.

Effect of excess of iron oxide and lead oxide on the microstructure and dielectric properties of lead-iron tungstate ceramics

Abstract: Lead-iron tungstate Pb(Fe2/3W1/3)O3; PFW) perovskite ceramics were prepared by the conventional mixed oxides method. Additional amounts of Fe2O3 and PbO were used to examine the role of excess oxides on the phase development, densification behaviour and dielectric properties. The stoichiometric mixture of the oxides results in a residual phase (Pb2WO5) besides the perovskite. Excess Fe2O3 eliminates the Pb2WO5, with a marked increase in the dielectric constant, whereas excess PbO leads to an unexpected increase of the amount of the second phase and a consequent decrease of the dielectric constant. The densification behaviour and the microstructures obtained after firing were very dependent on the starting stoichiometry. The PFW phase obtained using PbO or Fe2O3 excesses shows various degrees of ordering which seem to be correlated with the presence of either W6+ vacancies or of Pb2+ vacancies.

Temperature-programmed reduction and oxidation of nickel supported on WO3–Al2O3 composite oxides

Abstract: A series of Ni/WO3–Al2O3 catalysts have been prepared by systematically varying the weight loading of WO3–Al2O3 and the pH during adsorption/impregnation. The catalysts developed an increased interaction between the impregnated nickel and the supported metal oxide; the charge on the mixed oxide resulted in the impregnated nickel cation to be preferentially adsorbed onto tungsten sites of the composite supports. The amount of nickel associated with tungsten oxide was found to be a function of both the tungsten oxide weight loading and the impregnation pH. An increase in the negative charge due to hydrolysis of the tungsten oxide phase in the aqueous impregnant is proposed to result in a tungsten oxide associated nickel that is twice as large for the catalysts prepared at pH 6 than that at pH 4. The Ni/support interaction formed during adsorption/impregnation and drying were studied by temperature-programmed reduction (TPR) and temperature-programmed oxidation (TPO). The tungsten oxide and associated nickel were found to form a nickel tungstate compound which reacted with H2 at ca. 1050 K but could be regenerated by TPO. An Ni/Al2O4 type surface compound was also formed that was reduced at ca. 1150 K.

Chemistry of surface tungsten species on tungsten trioxide/alumina composite oxides under aqueous conditions

Abstract: The method of potentiometric titration in conjunction with an improved procedure for smoothing the experimental data and deconvoluting the proton binding isotherms was used to characterize the proton binding processes at the oxide/solution interface for Al[sub 2]O[sub 3], WO[sub 3] and a series of WO[sub 3]/Al[sub 2]O[sub 3] composite oxides. A similar procedure was applied to reference tungsten compounds (Ba WO[sub 4], ammonium metatungstate) for which the buffering capacity due to proton consumption in hydrolysis equilibria was characterized. The results showed that the reactivity towards protons of WO[sub 3]/Al[sub 2]O[sub 3] composites with low WO[sub 3] loading is much closer to that of the alumina support, while for high WO[sub 3] loading it is better described by the aqueous chemistry of tungstate/isopolytungstate solutions rather than of crystalline WO[sub 3]. At surface densities close or above monolayer, supported tungsten species showed a definite tendency to form polymeric surface aggregates in solutions with pH < 6 and more extensively below pH 4. The consequences of these phenomena on the equilibrium adsorption of cationic activators (Ni, Co) on preformed WO[sub 3]/Al[sub 2]O[sub 3] supports are discussed. 30 refs., 4 figs.

Anti corrosion treatment of aluminium or aluminium alloy surfaces

Abstract: An anti corrosion treatment of an aluminium or aluminium alloy surface comprising the steps of treating the surface with a solution having a pH of from greater than 0 to less than 2 and comprising at least one metal oxo ion completed with phosphorus (V) wherein the metal oxo ion is vanadate or tungstate or a mixture thereof.

FT-IR Emission Spectroscopic Studies of the Thermal Decomposition of Ammonium Molybdate, Vanadate, and Tungstate

Abstract: The thermal decomposition of ammonium molybdate, vanadate, and tungstate has been studied by in situ Fourier transform infrared (FT-IR) emission spectroscopy and differential thermal analysis (DTA) in the temperature range of 100-500°C. The results indicated that ammonium vanadate directly converted into V2O5 with the simple loss of its ammonium ion at around 230°C; however, the ammonium ion of molybdate and tungstate decomposed in two steps at 230 and 330°C, respectively. The emission spectra are presented with a relative emittance value which was directly calculated from the single-beam emission spectra of the sample and reference at corresponding temperatures. The emission spectra in relative emittance were proved to be temperature independent and free of the distortion usually caused by radiation and absorption of the instrument and water vapor in the optical path. The study demonstrates that FT-IR emission spectroscopy is a potentially useful technique for the characterization of catalyst preparation.

A factor found in aged tungstate solution enhanced the antibacterial effect of β-lactams on methicillin-resistant Staphylococcus aureus

Abstract: We have found a factor (Factor T) in aged mixtures of tungstate and phosphate which greatly enhances the antibacterial effects of beta-lactams on both inducible and constitutive methicillin-resistant Staphylococcus aureus, but not on methicillin-susceptible S. aureus. Factor T alone did not strongly inhibit bacterial growth. There was no synergism of Factor T with other classes of antibiotics, nor with other groups of bacteria, and it reduced the efficacy of amino-glycosides and tetracycline. Upon preparation of Factor T, acidifying and heating the mixture of tungstate and phosphate resulted in a high yield and reproducibility, and no enhancing effect was observed when other anions such as sulfate or molybdate were used instead. Factor T is heat- and acid-stable but labile to alkalization, and is probably a complex of phosphate and tungstate.

Molybdenum(VI) and tungsten(VI) complex formation. Part 5. The reaction with lactate in 1.0 mol dm–3 sodium chloride medium

Abstract: Complex formation of molybdate and of tungstate with lactate have been investigated in the pHc range 2–7.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm–3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account the side-reactions of molybdate and tungstate with hydrogen ions. For both systems the ‘best’ reaction model comprises only one major complex and a number of minor complexes. Thermodynamic quantities have been determined for each of the major complexes. For the molybdenum(VI) complex, [MoO2(C3H4O3)2]2–, the values for the formation constant and enthalpy change are log β122= 15.71 and ΔH⊖=–72 kJ mol–1 respectively and for the tungsten(VI) complex, [WO2(C3H4O3)2]2–, the values are log β122= 17.47 and ΔH⊖=–80 kJ mol–1. The equilibrium constant and enthalpy change for the protonation of lactate have also been determined, log K= 3.60 and ΔH⊖=–1.7 kJ mol–1. The energetics of complexation is discussed in terms of the thermodynamic quantities.

Crystal structure of tetraphenylarsonium tris(benzenedithiolato)tungstate, (As(C6H5)4)2(W(C6H4S2)3)

Abstract: Crystallization in monoclinic C12/c1 with lattice parameters a = 24.67 (1), b = 13.806 (6), c = 18.596 (6) A, β = 116.87°, T = 200 K, Z = 4; the final R value is 0.067.

Evidence for rapid ligand redistribution in non-aqueous tungstate chemistry: rational synthesis of the binuclear tungsten oxoalkoxide [W2O5(OMe)4]2–

Abstract: In the solid state, the tungstate [NMe3(CH2Ph)]2[WO4]·H2O contains chains of alternating [WO4]2– anions and water molecules linked by WO ⋯ H-O-H hydrogen bonding; in organic solvents, oxo-alkoxo ligand exchange occurs readily between Q2WO4(Q+= NBun4+ or [NMe3(CH2Ph)]+) and [{WO(OMe4)}2] to produce the new binuclear oxoalkoxo-tungstate [W2O5(OMe)4]2–, which is a precursor for non-aqueous polyoxo-tungstate synthesis.

Transition metal ions in silicate melts. III: Nickel in quenched oxide glasses

Abstract: Optical absorption spectra characteristic of Ni 2+ in octahedral coordination have been measured on glasses quenched from nickel bearing sodium silicate melts. The spectra are insensitive to changes in Na 2 O/SiO 2 ratio and are interpreted in terms of Ni 2+ as a free ion in a large open site rather than as Ni 2+ in a tetrahedral site. Raman spectra also indicate that increasing the NiO concentration increases the concentration of non-bridging oxygens. Comparison of Ni 2+ spectra in germanate, silicate, borate, tungstate and phosphate glasses suggests that Ni 2+ is in sixfold coordination in all of the glasses but that the values of the optical parameters Dq and B are linearly related and span a wide range of values

Sinterverbundwerkstoff fuer elektrische kontakte in schaltgeraeten der energietechnik

Abstract: Contact materials with the composition AgSnO2Bi2O3CuO have proved their worth, especially for low-voltage switching devices. Other metal oxides may be added to these materials. Besides the required useful life and a suitable over-temperature behaviour, such materials are also required to meet all requirements under short-circuit current stress. This is achieved by the invention in that the other metal oxide is separately added stannic oxide and/or iron and contains at least one element of the sixth sub-group of the periodic system. The other metal oxide of the sintered composite material of the invention is in particular ferric tungstate (FeWO4). Preferably, there is separately mixed SnO2 in combination with FeWO4. To produce such a sintered composite material, an internally oxidised alloy powder is made from an AgSnBiCu alloy and mixed with the powder of the other metal oxide or oxides and the powder mix is sintered into mouldings with a residual porosity

Synthesis of multifunctional nanocomposite material: tungstate-pillared Mg-Al hydrotalcite-like compound

Abstract: Hydrotalcite-like compounds (HTLC) with different cationic ratios (Mg/AI) and charge-balancing inter layer anions were prepared and characterized. The surface areas of HTLCs were found to increase with increasing Mg/AI ratio. Pure HTLCs were obtained when the Mg/AI ratio was below 7. Water adsorption properties of HTLCs show that they are microporous. BET surface areas with water and nitrogen are dependent on the Mg/AI ratio of HTLC. Tungstate-pillared HTLC (synthesized from Mg/AI=7) was prepared and characterized, resulting in higher gallery height and surface area.

Preparation of Rare Earth Oxide Dispersion Tungsten Powder by Spray Dry Method and It's Sintering

Abstract: A new method was developed to prepare rare earth oxide dispersion tungsten powder, and some sintering characteristics of the powder were investigated. The results are as follows :(1) Rare earth oxide dispersion tungsten powder can be prepared by spraying an acetic acid aqueous solution of ammonium tungstate para pentahydrate and lanthanum (III) oxide.(2) Rare earth oxide is dispersed as fine particles in tungsten matrices of sintered specimens. This finding contrasts with the fact that the conventional V-Blender mixing gives sintered specimens in which rare erath oxide is dispersed as coarse particles at the grain boundaries. Therefore, the spray dry method is a hopeful process to produce rare earth oxide dispersion tungsten alloys.

Kinetics and Mechanism of Reactions of Silver Tungstate with Mercuric Bromoiodide and Mercuric Chlorobromide in the Solid State.

Abstract: The kinetics and mechanism of the Ag 2 WO 4 -HgBrI and Ag 2 WO 4 -HgClBr reactions were studied by X-ray diffraction, thermal and electrical conductivity measurements. The kinetics were studied at several temperatures and for different lengths of air- gap between the two reactants. The kinetic data fit the equation X n = Kt , where X is the thickness of the product layer at time t and n and K are constants. Silver tungstate reacts with mercuric bromoiodide and mercuric chlorobromide in the molar ratio 1:1 to give Ag(Br,I), HgWO 4 and Ag(Cl,Br), HgWO 4 , respectively, as the end products.

Synthesis and Characterization of a New Ternary Transition Metal Nitride, FeWN2, from the Transition Metal Oxide Precursor, FeWO4

Abstract: The ternary nitride, FeWN2, has been synthesized via ammonolysis of the transition metal tungstate, FeWO4. FeWN2 is hexagonal with a unit cell of a = 2.8724(1) A and c =10.973(4) A. Rietveld refinements were performed in the space group P3-1C with Rwp=9.91%, and Rp=7.23%. The structure consist of alternating octahedrally coordinated iron-nitrogen layers and trigonal prismatically coordinated tungsten-nitrogen layers. Mossbauer data at 250K indicates an isomer shift δ=0.85 and shows no quadrupole splitting, which is consistent with the octahedral iron coordination in the proposed structure.

Electron microscopic observation of as-grown composition modulations in some optical oxide single crystals

Abstract: In undoped and doped bismuth germanate, tellurium dioxide, zinc tungstate and lithium niobate single crystals grown from melts of congruent compositions as-grown, impurity-induced composition modulation of wavelengths between 1.5 and 225 nm could be observed by electron microscopy. A detailed study of Ti-and/or Mg-doped lithium niobate samples of different dopant concentrations showed that over a critical Ti concentration of about 0.25 mM/M the composition modulations extend from the microscopic to macroscopic dimensions of the crystals.

Sintered composite materials for electric contacts in power technology switching devices and process for producing them

Abstract: Contact materials with the composition AgSnO 2 Bi 2 O 3 CuO have proven themselves, particularly for low-voltage switching devices. These materials can contain an additive of other metal oxides. In addition to the required useful lifetime and suitable excess high temperature behavior, such materials are also supposed to fulfill all the requirements under short-circuit current stress. This is accomplished, according to the invention, in that the additional metal oxide is separately added stannic oxide and/or iron containing at least one element of the sixth sub-group of the periodic system. In the sintered composite material according to the invention, the other metal oxide is particularly ferric tungstate (FeWO 4 ). Preferably, separately added SnO 2 is present in combination with FeWO 4 . To produce such a sintered composite material, an internally oxidized alloy powder is produced from an AgSnBiCu alloy and mixed with the powder of the additional metal oxide or metal oxides, and the powder mixture is processed to yield molded elements with a residual porosity of less than 1.5%, using production steps from sintering technology.