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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Acousto-optic modulators are main component of communication fibre-optic systems. From the other side the studies of the absolute piezo-optic effect (POE) are one of the important steps to estimate the acousto-optic efficiency of optical materials.The investigation results of POE in calcium tungstate crystals (CaWO4) has been represented. It is also proved that this crystal is essentially better acousto-optic material in comparison with lithium niobate (LiNbO3) widely used in acousto-optic devices.

 

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Because there are Joule heat and arc high temperature function between tungsten silver contacts, so their surface will reach a molten state in the opening and closing moment, which will be solidified in the cooling process and weld after that, leading a working contact no longer disconnected. If machinery is poorly designed or prepared improperly, it can cause silver tungsten contact bounce and short arc, at this time, its welding problem will be very prominent.

Anti-welding properties of silver tungsten contacts depend on the welding force of silver tungsten contacts; welding force refers to the force needed to be applied to separate the two already welded silver tungsten contacts, its size is related to the tensile strength of silver tungsten contacts material. If the contact material’s toughness is better, the tensile strength of silver tungsten contacts will be stronger, and the welding force will be greater, and the worse of its anti-welding properties; on the contrary, if the tensile strength of silver tungsten contacts is smaller, and the welding force will be smaller, and the better of its anti-welding properties.



 

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Notes of silver tungsten points prepared by infiltration are shown as follows:

1. The thickness of the tungsten powder. The closed pores inside tungsten porous skeleton will increase and then influence silver (liquid metal)’s entry if tungsten powder is too thin.

2. The structure of tungsten skeleton. The silver tungsten points could get the best electrical contact performance as the porous tungsten skeleton has firm and interconnected network structure. Besides, it can form the desired porous tungsten skeleton structure when the tungsten powder possesses a certain particle size and particle size composition. It is a network of firm and interconnected tungsten particles and has open pores with smooth surface.

3. Infiltration temperature. Infiltration temperature will have an effect on the quality of silver tungsten points. The viscosity of the liquid will be reduced to benefit infiltration if the temperature is too high. However, it is detrimental to the infiltration because the surface tension of the liquid is also reduced. The porous tungsten skeleton strength after sintering will decrease if the temperature is too low. And you should prolong the sintering time to make it be conducive to economic production.



 

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