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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Matters needing attention of silver tungsten contacts prepared by infiltration are the following:

1. Tungsten powder’s thickness. If tungsten powder is too thin, then, the closed pores inside tungsten porous skeleton will increase, affecting silver (liquid metal) to enter.

2. The structure of tungsten skeleton. Only when the porous tungsten skeleton has interconnected and firm network structure, the prepared silver tungsten contacts can get the best electrical contact performance. Besides, only the tungsten powder having a certain particle size and particle size composition, it will be able to form the desired porous tungsten skeleton structure. It is a network of interconnected and solid particles consisting of tungsten particle and has open pores with smooth surface.

3. Temperature. Temperature will affect the quality of silver tungsten contacts. If the temperature is too high, the viscosity of the liquid will be reduced in favor of infiltration, however, because the surface tension of the liquid is also reduced, so that is detrimental to the infiltration. If the temperature is too low, and after sintering, the porous tungsten skeleton strength decreases, you need to extend the sintering time and is not conducive to economic production.



 

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Measurements of the luminescence decay time have been made for a number of single crystals of calcium tungstate for excitation by cathode rays, α and γ radiation. The values obtained for cathode-ray excitation were, in general, 20–30% higher for all crystals, while for α excitation, several crystals showed no change in decay time, while others showed a decay time 20% faster. Measurements of the temperature dependence of the decay time and thermoluminescence experiments indicate that these differences in behavior can be attributed to differences in the density of energy traps and nonradiative centers in the crystal and to a rise in temperature in the excited channel. μ Measurements of the luminescence decay time have been made for a number of single crystals of calcium tungstate for excitation by cathode rays, α and γ radiation. The value of the decay time was found to depend both on the crystal used and on the nature of the excitation. For γ-ray excitation, the decay time was in the range 6.1 to 6.8.

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Scheelite mineral (CaWO₄) is the main raw material used in tungsten production. In this study the dissolution of synthetically prepared CaWO₄ in oxalic acid (H₂C₂O₄) solutions was studied. The effect of following parameters on the dissolution of reaction was investigated: stirring speed, temperature, H₂C₂O₄ concentration and particle size. The amounts of tungsten leached were followed by using Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES) instrument. X-ray Powder Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analytical techniques were used to characterize the leach residues. The results obtained show that the dissolution reaction takes place in two steps: first, the intermediate calcium aqua oxalato tungstate (Ca[WO₃(C₂O₄)H₂O]) chelate compound was obtained. Second, this compound reacted with H₂C₂O₄ to form water soluble hydrogen aqua oxalato tungstate (H₂[WO₃(C₂O₄)H₂O]) and solid CaC₂O₄H₂O. CaC₂O₄H₂O formed covered the surfaces of unreacted CaWO₄particles as a protective layer and the dissolution reaction was interrupted after a certain conversion at high temperatures and H₂C₂O₄ concentrations.

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