Tungsten silicide is a black and gray (blue gray) tetragonal crystals are stable at room temperature and atmospheric pressure, its physical properties are as follows: chemical formula: WSi2, molecular weight: 240.01, melting point:> 900 ℃, tungsten silicide insoluble in water and aqua regia, can be dissolved in a mixture of nitric acid and hydrofluoric acid. Tungsten silicide is usually used for making anti-oxidation coating layer and the resistance wire poultice. It has a certain irritation, if you do not accidentally come into contact the tungsten silicide may cause inflammation of skin and eyes, although it is usually not hazardous to water, but the absence of relevant government departments licensing, the material can not be discharged into the surrounding environment. For storage of silicon oxide, it should be sealed and placed in a room temperature, cool and dry place.
Tungsten silicide related system numbering: CAS registry number: 12039-88-2, MDL number: MFCD00049704, EINECS number: 234-909-0, PubChem ID: 24864814.

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In 1950s, tungsten powder is often prepared by tungsten acid and yellow tungsten oxide. In 1960-1970s, America, France, Netherlands and Britain used blue tungsten oxide to produce tungsten powder instead of tungsten acid and yellow tungsten oxide. China started to do the research on blue tungsten oxide in 1970-80s. Tungsten powder property is being affected by a lot of factors including producing method and raw material. When producing super fine tungsten powder, the oxide content of tungsten oxide should be low. When producing fine tungsten powder, oxide content of tungsten oxide should be higher. Thus the producing method of blue tungsten oxide which is of low cost, high property and environmental friendly is under investigation.

Use ammonium paratungstate as raw material, under specific preparing technology and certain temperature, ammonia cracked into hydrogen, ammonium paratungstate is under micro reduction and then blue tungsten oxide is prepared.




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Ammonium paratungstate preparing blue tungsten oxide affecting factors are listed as below:

1.Temperature. As the temperature going up, grain size of blue tungsten oxide changes a little and irregularly. It turns out the effect of temperature is not obvious. Oxide content and ammonia content fall down.

2.Feeding speed. Feeding speed mainly controls the reduction time. The faster the feeding speed is, the shorter the reduction time. As the prolong of reduction time, oxide and ammonia content reduces. But the temperature has larger influence than reduction time. Ammonium paratungstate cracks into crystal water and ammonia under high temperature and turns into yellow tungsten oxide. Yellow tungsten oxide which is under hydrogen reduction and turns into blue tungsten oxide.

3.Speed of rotary furnace. As the speed of rotary furnace goes up, blue tungsten oxide grain size will be more even. It is because material gets more contact with furnace and the heating surface is bigger. It increase the uniformity and distribution of grain size of blue tungsten oxide.




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Samples of tungsten-modified borated zirconia were prepared to be used as catalyst in 2-phenoxyethanol acetylation. The borated zirconias were obtained by impregnation of two zirconias (S2 and S4) obtained by the micellar method using different zirconium precursor concentrations with boric acid solution containing 0.3 moles B/dm³. These solids and the parent zirconias were impregnated with two concentration levels of ammonium metatungstate solutions (0.11 and 0.22 moles W/dm³). The solids containing boron mainly have amorphous characteristics and strong acidity, while those that contain only tungsten showed stronger acidity and evidence of the presence of a crystalline phase.

The specific surface area and the pore volume slightly decreased in the solids obtained by adding tungsten to borated zirconia, and more markedly in those that contain only tungsten, and the mean pore size increased with respect to the parent support. It was observed that in the 2-phenoxyethanol acetylation using acetic acid as acylating agent and tungsten-modified borated zirconia as catalyst, the yield to acetylated product was high and it is correlated with the acid strength of the catalysts. In turn, the solids that contain only tungsten have a correlation with the acidity, but lead to low yield values due to a poorer dispersion of the tungsten species. The best yield was obtained with the catalyst prepared with the borated S4 zirconia containing 15% g B₂O₃/100 g support and 12.5% g W/100 g support. Using this catalyst and the same reaction conditions, the acetylation of different alcohols and phenols was studied, the reactivity order being as follows: primary alcohols > secondary alcohols > phenols.


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At present both ammonium paratungstate (APT) and AMT are being used as tungsten sources in the catalyst industry. Since APT has a limited solubility in water (about 2 to 3 percent at room temperature), the user converts APT to a more soluble peroxytungstate form by means of reaction with hydrogen peroxide in order to prepare the catalyst. While AMT is highly water soluble, and thus need not be converted, it is in general more expensive than APT, due to the complexity and/or low yields of processes for producing it.

A process for producing ammonium metatungstate (AMT) from ammonium tungstate (AT) solution is described which involves the addition of about 3.6 percent by weight of silica to an AT solution, digestion for at least about 4 hours at a temperature of at least about 98°C, followed by filtration to remove the silica from the AMT solution. Typically about 0.4 percent by weight of silica remains after filtering. The resulting AMT solution may be further processed to recover solid AMT, such as by evaporation or spray drying.


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A particular advantage of the invention is that the silica used in digestion and subsequently filtered from the digested solution of ammonium metatungstate may be reused repeatedly, that is, its capacity to promote formation of AMT is not diminished with use. While the exact mechanism is not well understood at this time, the role of silica is to prevent the precipitation of insoluble APT and therefore to allow lowering of the pH to the AMT forming region, about 4 to 4.5.

After digestion is completed, the silica is removed, such as by filtering, and the solid AMT may be recovered in the conventional manner, such as by evaporation or spray drying.

The silica used in the invention may be any commercially available silica, provided that its impurities, both in kind and amount, are consistent with the final envisioned application for the AMT. For example, both sodium and phosphorus may be considered objectionable impurities for certain catalyst applications. Residual silica which cannot be removed from the digested AMT solution by filtering is generally below 1 percent by weight, and typically from 0.3 to 0.6 weight percent WO₃basis, and is compatible with many catalyst applications.


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The purpose of this study is to establish the way in which different synthesis conditions influence on the structural and luminescent characteristics of europium activated calcium tungstate powder phosphor. CaWO₄:Eu³⁺ samples were prepared by thermal synthesis from mixtures consisting of precipitated-CaWO₄, equivalent amounts of Eu₂O₃ and WO₃ (activating system) and CaCl₂ or Na₂WO₄ as flux. Calcination was performed at 800 - 1000 degree(s)C for 2 h, in air. The crystalline structure (XRD-patterns) and luminescent characteristics (emission and excitation spectra of phosphor samples were determined and interpreted.


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The induced absorption spectra were measured for equally grown CaWO₄ and CaWO₄ : La single crystals after ⁶⁰Co radioisotope irradiation by doses within 1 to 230 Gy. La doping increases considerably the radiation hardness of CaWO₄ material similarly as was observed for isostructural PbWO₄ single crystals recently. Transmission of as grown undoped CaWO₄ shows a noticeable decrease below 450 nm, which gives the evidence of deep trapping states. These trapping centres are effectively diminished by La doping as well. Induced absorption bands are discussed in the light of known and hypothesized color centers in CaWO₄ and PbWO₄single crystals, which were reported earlier in the literature.

 

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Utilization of luminescent substances in various optoelectronic devices depends on their luminescent properties and sensitivity to various excitation radiation as well as on particle size distribution and crystalline structure of luminous powders. Calcium tungstate phosphors are well excited with roentgen radiation, so that they are largely used for manufacture of x-ray intensifying screens. Being sensitive to short UV-radiation as well, they could be utilized in Plasma Display Panels or in advertising signs fluorescent tubes. In order to diversify the utilization possibilities of this tungstate class, luminescent powders based on CaWO₄:Eu³⁺ and CaWO₄:Tb³⁺ were synthesized and characterized. As compared with the starting self-activated phosphor, larger excitation wavelength domain and emission colors from blue-to-green-to- yellow-to-red were obtained. The good UV excitability and variable luminescence color recommend these phosphors for optoelectronic device manufacture.

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The surface of silver tungsten points would reach a molten state in the opening and closing moment because there are Joule heat and arc high temperature action among tungsten silver points and which will be solidified in the cooling process and weld after that resulting that working contacts no longer disconnected. If machinery is designed poorly or prepared improperly, which can bring about the result of silver tungsten contact bounce and short arc, and its welding problem will be very prominent at the time.

Anti-welding properties of silver tungsten points lie on the welding force of silver tungsten points. Welding force is referring to the force needed to separate the two welded silver tungsten points, and its size is connected to the tensile strength of silver tungsten points. If the toughness of contact material is better, then the tensile strength of silver tungsten points will be stronger, and the welding force will be greater, and its anti-welding properties will be worse; on the contrary, if the tensile strength of silver tungsten points is weaker, and the welding force will be smaller, and its anti-welding properties will be better.



 

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