Showing papers on "Tungstate published in 1974"

Luminescence of calcium tungstate crystals

Abstract: Fluorescence, excitation, and pulsed fluorescence measurements have been made on a single crystal of CaWO4 over a wide temperature range. The temperature dependences of the integrated fluorescence intensities and lifetimes were obtained for different excitation wavelengths. A sharp increase in the fluorescence lifetimes is observed below about 75°K and zero‐phonon lines can be observed. These results are used to formulate a model for the tungstate luminescent center based on the WO42− ion in a crystal field of S4 symmetry. Energy migration and impurity trapping effects are also discussed.

Preparation of heteropoly acids of tungsten and molybdenum

Abstract: Heteropoly acids of tungsten and molybdenum, such as tungstosilicic acid and molybdosilicic acid, are produced by first preparing an aqueous solution of an alkali metal tungstate or molybdate and an alkali metal salt of the hetero atom, and then removing the alkali metal cations from the solution by means of a cation exchange membrane under the influence of an applied electrical potential, whereby the reactive tungstate or molybdate and hetero species anions react to form the desired heteropoly acid.

Process for producing ammonium metatungstate from ammonium tungstate by ion exchange

Abstract: A process for producing crystalline ammonium metatungstate (AMT) from ammonium tungstate (AT) solution by introducing AT solution into an ion exchange column containing a weak acid (carboxylic group) cation exchange resin, and collecting the effluent from the column until a pH of about 3.5 is reached. The effluent solution is then digested at about 98*C for about 5 hours, followed by crystallization of AMT by conventional processes, such as, evaporation or spray drying.

The Crystallisation of Calcium Molybdate and Tungstate from Lithium Chloride Melts by Continuous Cooling. The Kinetics of Crystal Growth in Alumina Crucibles

Abstract: The kinetics of crystallisation of calcium molybdate and tungstate from unstirred supersaturated solutions in lithium chloride melts — in alumina crucibles — was studied by continuous cooling from initial temperature T0 = 800°C down to room temperature at cooling rates RT = 20° to 200°C hr−1. The solutions were analysed chemically and the crystals were examined by optical microscopy. Crystal growth started practically immediately after the onset of cooling: at first, the amount of material deposited onto crystals was far less than the amount of excess solute developed within the supersaturated solutions but crystallisation rates then increased as the crystal sizes increased. Then, after some time t* (at about seventy percent crystallisation), all excess solute was deposited onto growing crystals.

Process for the manufacture of ammonium tungstate ammonium paratungstate, ammonium metatungstate and hydrated tungsten trioxide

Abstract: Ammonium tungstate, ammonium paratungstate, ammonium metatungstate or hydrated tungsten trioxide is produced by passing tungstate anions through an anion exchange membrane into an aqueous solution containing ammonium cations under the driving force of an electrical potential for a time sufficient to achieve a pH within the range in which the desired tungsten compound will form.

Conductance method for determining the mechanical properties of propellants

Abstract: A solid propellant grain is converted from a non-conductive filled elasto into a highly-conductive filled elastomer by introducing an effective amount of an alkali metal tungstate or the analogous ammonium salt which, in addition to imparting electrical conductance characteristics, also functions as an oxidizer in the propellant composition. When sodium tungstates, known as tungsten bronzes, or the analogous ammonium salt, are used to replace from 1 to 3 parts of the ammonium perchlorate oxidizer in the propellant formulation, it changes the grain to a highly-conductive grain, and, thereby, permits measuring the continuity of the propellant grain by means of electrical conductance measurements. These conductance measurements provide means of assessing the changes which take place in the solid propellant grain, such as, the physical and chemical changes (which affect mechanical properties) which the grain has undergone during storage or aging or when the grain is subjected to different load conditions, such as, acceleration, blast, shock, etc. In addition to the benefit of the reliable means for assessing such changes, the propellant grains employing the tungstates show an unpredicted increase in the propellant burning rate. The propellant density also increased due to the high density of the tungsten bronzes, and this is another benefit.

Tungsten trioxide coatings for luminescent materials - suitable for use with zinc silicate activated with manganese

Abstract: In a luminescent matl. enveloped with an oxide and used for cathode ray tubes, the oxide coating is tungsten VI oxide (WO3) deposited at 100-1300 degrees C. The luminescent matl. is pref. a silicate, esp. zinc silicate matl., but it may be an oxide, phosphate, borate, vanadate, tungstate or sulphate luminescent matl. The pref. process consists of mixing the luminescent matl. with powdered WO3 and heating for 10 mins. to 10 hrs. at 100-1300 degrees C. The mixt. pref. contains 0.1-90, esp. 28-38 wt% WO3. Used for e.g. adjusting the threshold value of emission of zinc silicate luminescent matls. activated with Mn. Good reproducibility can be obtd. even in the case of large batches, and the threshold value can be adjusted by varying the amt. of WO3 employed.

Vinyl chloride polymers containing zinc tungstate

Abstract: Vinyl chloride polymers are compounded with zinc tungstate to provide improved flame resistance and reduce smoke production.

Electroanalytical and catalymetric microdeterminations of molybdates

Abstract: A comparative study devoted to the comparison of different methods for the microdetermination of molybdates has been achieved in this laboratory: (a) polarographic reduction of Mo(VI) in citric acid medium, (b) polarographic detection of Mo(VI) in the presence of H 2 O 2 , (c) coulometric reduction of Mo(VI) by cathodic generation of Ti(III), (d) cathodic deposition and anodic reoxidation of MoO 2 , (e) catalymetric determination of Mo(VI) acting as a catalyst in the homogeneous reduction of BrO 3 − by I − . These methods are compared in relation to the minimum detectable concentrations or quantities of Mo(VI) and the error limits are indicated. Interference of tungstates in method (d) is studied. Method (e) also allows accurate determinations of tungstates, especially if tungstate is present as paratungstate B ion. Methods (a) and (c) have been used by one of us (P. J.) for the determination of the thermodynamic solubility product of BaMoO 4 and of the mean activity coefficients of this salt in sodium chloride solutions of increasing ionic strength (0 to 5 M in NaCl).

Investigation of the interaction of molybdate and tungstate ions with water

Abstract: 1. The WO42su−1 ion forms stronger bonds with water in solution than the MoO42su−1 ion. 2. In the crystal hydrates Na2MO4 · 2H2O (M=Mo, W), the opposite picture is observed; this is evidently explained by structural changes, accompanying the formation of crystal hydrates.

Thermally opacifiable glasses

Abstract: This invention relates to thermally opacifiable glasses which exhibit uniformly dense opacity, that do not require the presence of lead or lithium, and contain as the primary opacifying phase a crystal selected from the group consisting of sodium molybdate, sodium tungstate, barium molybdate, barium tungstate, and mixtures thereof. The glasses consist essentially, by weight on the oxide basis, of about 64-80% SiO2, 2-12% Al2O3, 9-16% R2O, wherein R2O consists of 9-15% Na2O and 0-5% K2O, 1.2-4% RO3, wherein RO3 consists of 1.2-4% MoO3, or 1.6-4% WO3, or mixtures thereof in amounts equivalent to at least 1.2% MoO3, 0.5-3% F, and, preferably, up to 4% BaO.